Material properties of diamond From Wikipedia, the free encyclopedia

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This article addresses the material properties of diamond. For a broader discussion of diamonds, see diamond. For other uses of the word diamond, see diamond (disambiguation).

Diamond

An octahedral diamond crystal in matrix

General

Category Native Nonmetal, Mineral

Formula (repeating unit) Carbon (C)

Identification

Color Most often colorless to yellow or brown. Rarely pink, orange, green, blue, gray, or red.

Crystal habit

Octahedral, cubo-octahedral, spherical or cubic

Crystal system

Diamond cubic (a = 3.56683 Å)

Cleavage Octahedral; perfect and easy

Fracture Conchoidal

Mohs scale 10

hardness

Streak white

Diaphaneity Clear to not

Specific gravity 3.516–3.525

Refractive index

2.417

Pleochroism None

Fusibility Burns above 700 °C in air.

Solubility

Resistant to acids, but dissolves irreversibly in hot steel

Major varieties

Ballas

Spherical, radial structure, cryptocrystalline, opaque black

Bort

Poorly formed, cryptocrystalline, shapeless, translucent

Carbonado Massive, microcrystalline, opaque black

Diamond is the allotrope of carbon in which the carbon atoms are arranged in the specific type of cubic lattice called diamond cubic. Diamond is an optically isotropic crystal that is transparent to opaque. Owing to its strong covalent bonding, diamond is the hardest naturally occurring material known. Yet, due to important structural weaknesses, diamond's toughness is only fair to good. The precise tensile strength of diamond is unknown, however strength up to 60 GPa has been observed, and it could be as high as 90–225 GPa depending on the crystal orientation. The anisotropy of diamond hardness is carefully considered during diamond cutting. Diamond has a high refractive index (2.417) and moderate dispersion (0.044) properties which give cut diamonds their brilliance. Scientists classify diamonds into four main types according to the nature of crystallographic defects present. Trace impurities substitutionally replacing carbon atoms in a diamond's crystal lattice, and in some cases structural defects, are responsible for the wide range of colors seen in diamond. Most diamonds are electrical insulators but extremely efficient thermal conductors. Unlike many other minerals, the specific gravity of diamond crystals (3.52) has rather small variation from diamond to diamond.

Contents

• 1 Hardness and crystal structure • 2 Toughness • 3 Optical properties

o 3.1 Color and its causes o 3.2 Luster o 3.3 Fluorescence o 3.4 Optical absorption

• 4 Electrical properties • 5 Thermal conductivity • 6 Thermal stability • 7 See also • 8 References • 9 Further reading • 10 External links

Hardness and crystal structure Known to the ancient Greeks as ἀδάμας – adámas ("proper", "unalterable", "unbreakable")[1] and sometimes called adamant, diamond is the hardest known naturally occurring material, scoring 10 on the Mohs scale of mineral hardness. Diamond is extremely strong owing to the structure of its carbon atoms, where each carbon atom has four neighbors joined to it with covalent bonds. The material boron nitride, when in a form structurally identical to diamond (zincblende structure), is nearly as hard as diamond; a currently hypothetical material, beta carbon nitride, may also be as hard or harder in one form. It has been shown that some diamond aggregates having nanometer grain size are harder and tougher than conventional large diamond crystals, thus they perform better as abrasive material.[2][3] Owing to the use of those new ultra-hard materials for diamond testing, more accurate values are now known for diamond hardness. A surface perpendicular to the [111] crystallographic direction (that is the longest diagonal of a cube) of a pure (i.e., type IIa) diamond has a hardness value of 167 GPa when scratched with an nanodiamond tip, while the nanodiamond sample itself has a value of 310 GPa when tested with another nanodiamond tip. Because the test only works properly with a tip made of harder material than the sample being tested, the true value for nanodiamond is likely somewhat lower than 310 GPa.[2]

The precise tensile strength of diamond is unknown, however strength up to 60 GPa has been observed, and it could be as high as 90–225 GPa depending on the perfection of diamond lattice and on its orientation: Tensile strength is the highest for the [100] crystal direction (normal to the cubic face), smaller for the [110] and the smallest for the [111] axis (along the longest cube diagonal).[4]

Cubic diamonds have a perfect and easy octahedral cleavage, which means that they only have four planes—weak directions following the faces of the octahedron where there are fewer bonds—along which diamond can easily split upon blunt impact to leave a smooth surface. Similarly, diamond's hardness is markedly directional: the hardest direction is the diagonal on the cube face, 100 times harder than the softest direction, which is the dodecahedral plane. The octahedral plane is intermediate between the two extremes. The diamond cutting process relies heavily on this directional hardness, as without it a diamond would be nearly impossible to fashion. Cleavage also plays a helpful role, especially in large stones where the cutter wishes to remove flawed material or to produce more than one stone from the same piece of rough (e.g. Cullinan Diamond).[5]

Animation of the diamond's crystal structure.

Diamonds crystallize in the diamond cubic crystal system (space group Fd3m) and consist of tetrahedrally, covalently bonded carbon atoms. A second form called lonsdaleite, with hexagonal symmetry, has also been found, but it is extremely rare and forms only in meteorites or in laboratory synthesis. The local environment of each atom is identical in the two structures. From theoretical considerations, lonsdaleite is expected to be harder than diamond, but the size and quality of the available stones are insufficient to test this hypothesis.[6] In terms of crystal habit, diamonds occur most often as euhedral (well-formed) or rounded octahedra and twinned, flattened octahedra with a triangular outline. Other forms include dodecahedra and (rarely) cubes. There is evidence that nitrogen impurities play an important role in the formation of well-shaped euhedral crystals. The largest diamonds found, such as the Cullinan Diamond, were shapeless. These diamonds are pure (i.e. type II) and therefore contain little if any nitrogen.[5]

The faces of diamond octahedrons are highly lustrous owing to their hardness; triangular shaped growth defects (trigons) or etch pits are often present on the faces. A diamond's fracture may be step-like, conchoidal (shell-like, similar to glass) or irregular. Diamonds which are nearly round, due to the formation of multiple steps on octahedral faces, are commonly coated in a gum-like skin (nyf). The combination of stepped faces, growth defects, and nyf produces a "scaly" or corrugated appearance. Many diamonds are so distorted that few crystal faces are discernible. Some diamonds found in Brazil and the Democratic Republic of the Congo are polycrystalline and occur as opaque, darkly colored, spherical, radial masses of tiny crystals; these are known as ballas and are important to industry as they lack the cleavage planes of single-crystal diamond. Carbonado is a similar opaque microcrystalline form which occurs in shapeless masses. Like ballas diamond, carbonado lacks cleavage planes and its specific gravity varies widely from 2.9 to 3.5. Bort diamonds, found in Brazil, Venezuela, and Guyana, are the most common type of industrial-grade diamond. They are also polycrystalline and often poorly crystallized; they are translucent and cleave easily.[5]

Because of its great hardness and strong molecular bonding, a cut diamond's facets and facet edges appear the flattest and sharpest. A curious side effect of diamond's surface perfection is hydrophobia combined with lipophilia. The former property means a drop of water placed on a diamond will form a coherent droplet, whereas in most other minerals the water would spread out to cover the surface. Similarly, diamond is unusually lipophilic, meaning grease and oil readily collect on a diamond's surface. Whereas on other minerals oil would form coherent drops, on a diamond the oil would spread. This property is exploited in the use of so-called "grease pens," which apply a line of grease to the surface of a suspect diamond simulant. Diamond surfaces are hydrophobic when the surface carbon atoms terminate with a hydrogen atom and hydrophilic when the surface atoms

terminate with an oxygen atom or hydroxyl radical. Treatment with gases or plasmas containing the appropriate gas, at temperatures of 450 °C or higher, can change the surface property completely.[7] Naturally occurring diamonds have a surface with less than a half monolayer coverage of oxygen, the balance being hydrogen and the behavior is moderately hydrophobic. This allows for separation from other minerals at the mine using the so-called "grease-belt".[8]

Toughness

Diamonds in an angle grinder blade

Unlike hardness, which denotes only resistance to scratching, diamond's toughness or tenacity is only fair to good. Toughness relates to the ability to resist breakage from falls or impacts. Because of diamond's perfect and easy cleavage, it is vulnerable to breakage. A diamond will shatter if hit with an ordinary hammer. The toughness of natural diamond has been measured as 2.0 MPa m1/2, which is good compared to other gemstones, but poor compared to most engineering materials. As with any material, the macroscopic geometry of a diamond contributes to its resistance to breakage. Diamond has a cleavage plane and is therefore more fragile in some orientations than others. Diamond cutters use this attribute to cleave some stones, prior to faceting.[9][10]

Ballas and carbonado diamond are exceptional, as they are polycrystalline and therefore much tougher than single-crystal diamond; they are used for deep-drilling bits and other demanding industrial applications.[11] Particular faceting shapes of diamonds are more prone to breakage and thus may be uninsurable by reputable insurance companies. The brilliant cut of gemstones is designed specifically to reduce the likelihood of breakage or splintering.[5]

Solid foreign crystals are commonly present in diamond. They are mostly minerals, such as olivine, garnets, ruby, and many others.[12] These and other inclusions, such as internal fractures or "feathers", can compromise the structural integrity of a diamond. Cut diamonds that have been enhanced to improve their clarity via glass infilling of fractures or cavities are especially fragile, as the glass will not stand up to ultrasonic cleaning or the rigors of the jeweler's torch. Fracture-filled diamonds may shatter if treated improperly.[13]

Optical properties

Color and its causes

Synthetic diamonds of various colors grown by the high-pressure high-temperature technique, the diamond size is ~2 mm

Pure diamonds, before and after irradiation and annealing. Clockwise from left bottom: 1) initial (2×2 mm); 2–4) irradiated by different doses of 2-MeV electrons; 5–6) irradiated by different doses and annealed at 800 °C.

Main article: Crystallographic defects in diamond

Diamonds occur in various colors — black, brown, yellow, gray, white, blue, orange, purple to pink and red. Colored diamonds contain crystallographic defects, including substitutional impurities and structural defects, that cause the coloration. Theoretically, pure diamonds would be transparent and colorless. Diamonds are scientifically classed into two main types and several subtypes, according to the nature of defects present and how they affect light absorption:[5]

Type I diamond has nitrogen (N) atoms as the main impurity, at a concentration of up to 1%. If the N atoms are in pairs or larger aggregates, they do not affect the diamond's color; these are Type Ia. About 98% of gem diamonds are type Ia: these diamonds belong to the Cape series, named after the diamond-rich region formerly known as Cape Province in South Africa, whose deposits are largely Type Ia. If the nitrogen atoms are dispersed throughout the crystal in isolated sites (not paired or grouped), they give the stone an intense yellow or occasionally brown tint (type Ib); the rare canary diamonds belong to this type, which represents only ~0.1% of known natural diamonds. Synthetic diamond containing nitrogen is usually of type Ib. Type Ia and Ib diamonds absorb in both the infrared and ultraviolet region of the electromagnetic spectrum, from 320 nm. They also have a characteristic fluorescence and visible absorption spectrum (see Optical properties).[14]

Type II diamonds have very few if any nitrogen impurities. Pure (type IIa) diamond can be colored pink, red, or brown owing to structural anomalies arising through plastic deformation during crystal growth[15] — these diamonds are rare (1.8% of gem diamonds), but constitute a large percentage of Australian diamonds. Type IIb diamonds, which account for ~0.1% of gem diamonds, are usually a steely blue or gray due to boron atoms scattered within the crystal matrix. These diamonds are

also semiconductors, unlike other diamond types (see Electrical properties). Most blue-gray diamonds coming from the Argyle mine of Australia are not of type IIb, but of Ia type. Those diamonds contain large concentrations of defects and impurities (especially hydrogen and nitrogen) and the origin of their color is yet uncertain.[16] Type II diamonds weakly absorb in a different region of the infrared (the absorption is due to the diamond lattice rather than impurities), and transmit in the ultraviolet below 225 nm, unlike type I diamonds. They also have differing fluorescence characteristics, but no discernible visible absorption spectrum.[14]

Certain diamond enhancement techniques are commonly used to artificially produce an array of colors, including blue, green, yellow, red, and black. Color enhancement techniques usually involve irradiation, including proton bombardment via cyclotrons; neutron bombardment in the piles of nuclear reactors; and electron bombardment by Van de Graaff generators. These high-energy particles physically alter the diamond's crystal lattice, knocking carbon atoms out of place and producing color centers. The depth of color penetration depends on the technique and its duration, and in some cases the diamond may be left radioactive to some degree.[5][17]

Some irradiated diamonds are completely natural — one famous example is the Dresden Green Diamond.[18] In these natural stones the color is imparted by "radiation burns" (natural irradiation by alpha particles originating from uranium ore) in the form of small patches, usually only microns deep. Additionally, Type IIa diamonds can have their structural deformations "repaired" via a high-pressure high-temperature (HPHT) process, removing much or all of the diamond's color.[19]

Luster

A scattering of round-brilliant cut diamonds shows the many reflecting facets

The luster of a diamond is described as 'adamantine', which simply means diamond-like. Reflections on a properly cut diamond's facets are undistorted, due to their flatness. The refractive index of diamond (as measured via sodium light, 589.3 nm) is 2.417. Because it is cubic in structure, diamond is also isotropic. Its high dispersion of 0.044 (variation of refractive index across the visible spectrum) manifests in the perceptible fire of cut diamonds. This fire—flashes of prismatic colors seen in transparent stones—is perhaps diamond's most important optical property from a jewelry perspective. The prominence or amount of fire seen in a stone is heavily influenced by the choice of diamond cut and its associated proportions (particularly crown height), although the body color of fancy (i.e., unusual) diamonds may hide their fire to some degree.[17]

More than 20 other minerals have higher dispersion (that is difference in refractive index for blue and red light) than diamond, such as titanite 0.051, andradite 0.057, cassiterite 0.071, strontium

titanate 0.109, sphalerite 0.156, synthetic rutile 0.330, cinnabar 0.4, etc. (see dispersion).[20] However, the combination of dispersion with extreme hardness, wear and chemical resistivity, as well as clever marketing, determines the exceptional value of diamond as a gemstone.

Fluorescence

Fluorescence tests for impurities

A micrograph (top) and UV-excited photoluminescence (bottom) from a plate cut from a synthetic diamond (width ~3 mm). Most of yellow color and green emission originate from nickel impurities.

Diamonds exhibit fluorescence, that is, they emit light of various colors and intensities under long-wave ultra-violet light (365 nm): Cape series stones (type Ia) usually fluoresce blue, and these stones may also phosphoresce yellow, a unique property among gemstones. Other possible long-wave fluorescence colors are green (usually in brown stones), yellow, mauve, or red (in type IIb diamonds).[21] In natural diamonds, there is typically little if any response to short-wave ultraviolet, but the reverse is true of synthetic diamonds. Some natural type IIb diamonds phosphoresce blue after exposure to short-wave ultraviolet. In natural diamonds, fluorescence under X-rays is generally bluish-white, yellowish or greenish. Some diamonds, particularly Canadian diamonds, show no fluorescence.[14][17]

The origin of the luminescence colors is often unclear and not unique. Blue emission from type IIa and IIb diamonds is reliably identified with dislocations by directly correlating the emission with dislocations in an electron microscope.[22] However, blue emission in type Ia diamond could be either due to dislocations or the N3 defects (three nitrogen atoms bordering a vacancy).[23] Green emission in natural diamond is usually due to the H3 center (two substitutional nitrogen atoms separated by a vacancy),[24] whereas in synthetic diamond it usually originates from nickel used as a catalyst (see figure).[14] Orange or red emission could be due to various reasons, one being the nitrogen-vacancy center which is present in sufficient quantities in all types of diamond, even type IIb.[25]

Optical absorption

Cape series (Ia) diamonds have a visible absorption spectrum (as seen through a direct-vision spectroscope) consisting of a fine line in the violet at 415.5 nm; however, this line is often invisible until the diamond has been cooled to very low temperatures. Associated with this are weaker lines at 478 nm, 465 nm, 452 nm, 435 nm, and 423 nm. All those lines are labeled as N3 and N2 optical centers and associated with a defect consisting of three nitrogen atoms bordering a vacancy. Other stones show additional bands: brown, green, or yellow diamonds show a band in the green at 504 nm (H3 center, see above),[24] sometimes accompanied by two additional weak bands at 537 nm and 495 nm (H4 center, a large complex presumably involving 4 substitutional nitrogen atoms and 2 lattice vacancies).[26] Type IIb diamonds may absorb in the far red due to the substitutional boron, but otherwise show no observable visible absorption spectrum.[5]

Gemological laboratories make use of spectrophotometer machines that can distinguish natural, artificial, and color-enhanced diamonds. The spectrophotometers analyze the infrared, visible, and ultraviolet absorption and luminescence spectra of diamonds cooled with liquid nitrogen to detect tell-tale absorption lines that are not normally discernible.[5][27]

Electrical properties Main article: Covalent superconductors

Except for most natural blue diamonds, which are semiconductors due to substitutional boron impurities replacing carbon atoms, diamond is a good electrical insulator, having a resistivity of 100 GΩ·m to 1 EΩ·m[28] (1011 to 1018 Ω·m). Natural blue or blue-gray diamonds, common for the Argyle diamond mine in Australia, are rich in hydrogen; these diamonds are not semiconductors and it is unclear whether hydrogen is actually responsible for their blue-gray color.[16] Natural blue diamonds containing boron and synthetic diamonds doped with boron are p-type semiconductors. N-type diamond films are reproducibly synthesized by phosphorus doping during chemical vapor deposition.[29] Diode p-n junctions and UV light emitting diodes (LEDs, at 235 nm) have been produced by sequential deposition of p-type (boron-doped) and n-type (phosphorus-doped) layers.[30] Diamond transistors have been produced.

In April 2004, the journal Nature reported that below the superconducting transition temperature 4 K, boron-doped diamond synthesized at high temperature and high pressure is a bulk superconductor.[31] Superconductivity was later observed in heavily boron-doped films grown by various chemical vapor deposition techniques, and the highest reported transition temperature (by 2009) is 11.4 K.[32][33]

Thermal conductivity Unlike most electrical insulators, diamond is a good conductor of heat because of the strong covalent bonding within the crystal. Most natural blue diamonds contain boron atoms which replace carbon atoms in the crystal matrix, and also have high thermal conductance. Thermal conductivity of natural diamond was measured to be about 22 W/(cm·K), five times more than copper. Monocrystalline synthetic diamond enriched in 12C isotope (99.9%) has the highest thermal conductivity of any known solid at room temperature: 33.2 W/(cm·K).[34][35] Because diamond has such high thermal conductance it is already used in semiconductor manufacture to prevent silicon and other semiconducting materials from overheating. At lower temperatures conductivity becomes

even better as its Fermi electrons can match the phononic normal transport mode near the Debye point,[36] and thermal conductivity reaches 410 W/(cm·K) at 104 K (12C-enriched diamond).[35]

Diamond's high thermal conductivity is used by jewelers and gemologists who may employ an electronic thermal probe to separate diamonds from their imitations. These probes consist of a pair of battery-powered thermistors mounted in a fine copper tip. One thermistor functions as a heating device while the other measures the temperature of the copper tip: if the stone being tested is a diamond, it will conduct the tip's thermal energy rapidly enough to produce a measurable temperature drop. This test takes about 2–3 seconds. However, older probes will be fooled by moissanite, a crystalline mineral form of silicon carbide introduced in 1998 as an alternative to diamonds, which has a similar thermal conductivity.[5][27]

Thermal stability

Diamond and graphite are two allotropes of carbon: pure forms of the same element that differ in structure.

Being a form of carbon, diamond oxidizes in air if heated over 700 °C.[37] In absence of oxygen, e.g. in a flow of high-purity argon gas, diamond can be heated up to about 1700 °C.[38][39] Its surface blackens, but can be recovered by re-polishing. At high pressure (~20 GPa) diamond can be heated up to 2500 °C,[40] and a report published in 2009 suggests that diamond can withstand temperatures of 3000 °C and above.[41]

Diamonds are carbon crystals that form deep within the Earth under high temperatures and extreme pressures. At surface air pressure (one atmosphere), diamonds are not as stable as graphite, and so the decay of diamond is thermodynamically favorable (δH = −2 kJ / mol).[17] So, contrary to De Beers' ad campaign extending from 1948 to at least 2006 under the slogan "A diamond is forever",[42] diamonds are definitely not forever. However, owing to a very large kinetic energy barrier, diamonds are metastable; they will not decay into graphite under normal conditions.[17]

Diamond enhancements are specific treatments, performed on natural diamonds (usually those already cut and polished into gems), which are designed to improve the gemological characteristics — and therefore the value — of the stone in one or more ways. These include clarity treatments such as laser drilling to remove inclusions, application of sealants to fill cracks, color treatments to improve a white diamond's color grade, and treatments to give fancy color to a white or off-color diamond.

The CIBJO and government agencies such as the United States Federal Trade Commission explicitly require the disclosure of most diamond treatments at the time of sale. Some treatments,

particularly those applied to clarity, remain highly controversial within the industry — this arises from the traditional notion that diamond holds a unique or "sacred" place among the gemstones, and should not be treated too radically, if for no other reason than a fear of damaging consumer confidence.

Treated diamonds usually trade at a significant discount to untreated diamonds. This is due to several factors, including relative scarcity — a much larger number of stones can be treated to reach gem quality than are found naturally occurring in a gem quality state — and the potential impermanence of various treatments. Therefore, it is unusual to see a diamond with good overall gemological characteristics undergo treatment. Diamonds which are chosen for treatment are usually those that would be otherwise difficult to sell as gem diamonds, where inclusions or fractures noticeably detract from the beauty of the diamond to even casual observers. In these cases, the loss in value due to treating the diamond is more than offset by the value added by the mitigating of obvious flaws.

Contents • 1 Clarity enhancements

o 1.1 Laser drilling o 1.2 Fracture filling

• 2 Color enhancements o 2.1 Irradiation o 2.2 Coatings o 2.3 High-pressure high-temperature treatment

• 3 See also • 4 Footnotes • 5 References

Clarity enhancements See also: diamond clarity

This section does not cite any references or sources. (July 2010)

The clarity, or purity, of a diamond — the relative or apparent severity of flaws within the stone — has, like the other "four Cs", a strong bearing on the evaluation of a diamond's worth. The most common flaws, or inclusions, seen in diamonds are fractures (commonly called feathers, due to their feathery whitish appearance), and solid foreign crystals within the diamond; such as garnet, diopside, or even other diamonds. The size, color, and position of inclusions can reduce the value of a diamond, especially when other gemological characteristics are good. Those who prepare diamonds for sale sometimes choose to reduce the visual impact of inclusions through one or more of a variety of treatments.

Laser drilling

The combustibility of diamond has allowed the development of laser drilling techniques which, on a microscopic scale, are able to selectively target and either remove or significantly reduce the visibility of crystal or iron oxide-stained fracture inclusions. Diamonds have been laser-drilled since at least the mid-1980s. Laser drilling is often followed by glass infilling.

The drilling process involves the use of an infrared laser (wavelength about 1060 nm) to bore very fine holes (less than 0.2 millimeters or 0.005 inches in diameter) into a diamond to create a route of access to an inclusion. Because diamond is transparent to the wavelength of the laser beam, a coating of amorphous carbon or other energy-absorbent substance is applied to the surface of the diamond to initiate the drilling process. The laser then burns a narrow tube to the inclusion. Once the included crystal has been reached by the drill, the diamond is immersed in sulfuric acid to dissolve the crystal or iron oxide staining. This process is not effective for inclusions which are diamonds themselves, as diamond is not soluble in sulfuric acid.

Several inclusions can be thus removed from the same diamond, and under microscopic inspection the fine bore holes are readily detectable. They are whitish and more or less straight, but may change direction slightly, and are often described as having a "wrinkled" appearance. In reflected light, the surface-reaching holes can be seen as dark circles breaching the diamond's facets. The diamond material removed during the drilling process is destroyed, and is often replaced with glass infilling, using the fracture filling techniques described below.

Fracture filling

Around the same time as the laser drilling technique was developed, research began on the fracture filling of diamonds to better conceal their flaws. The glass filling of diamond often follows the laser drilling and acid-etching of inclusions, though if the fractures are surface-reaching, no drilling may be required. This process, which involves the use of specially-formulated glasses with a refractive index approximating that of diamond, was pioneered by Zvi Yehuda of Ramat Gan, Israel. Yehuda is now used as a brand name applied to diamonds treated in this manner,[1] and the process has apparently changed little since its inception. Koss & Schechter, another Israel-based firm, attempted to modify Yehuda's process in the 1990s by using halogen-based glasses, but this was unsuccessful. The details behind the Yehuda process have been kept secret, but the filler used is reported to be lead oxychloride glass, which has a fairly low melting point. The New York-based Dialase also treats diamonds via a Yehuda-based process, which is believed to use lead-bismuth oxychloride glass.

The glass present in fracture-filled diamonds can usually be detected by a trained gemologist under the microscope: the most obvious signs — apart from the surface-reaching bore holes and fractures associated with drilled diamonds — are air bubbles and flow lines within the glass, which are features never seen in untreated diamond. More dramatic is the so-called "flash effect", which refers to the bright flashes of color seen when a fracture-filled diamond is rotated; the color of these flashes ranges from an electric blue or purple to an orange or yellow, depending on lighting conditions (light field and dark field, respectively). The flashes are best seen with the field of view nearly parallel to the filled fracture's plane. In strongly colored diamonds the flash effect may be missed if examination is less than thorough, as the stone's body color will conceal one or more of the flash colors. For example, in brown-tinted "champagne" diamonds, the orange-yellow flashes are concealed, leaving only the blue-purple flashes to be seen. One last but important feature of fracture-filled diamonds is the color of the glass itself: it is often a yellowish to brownish, and along with

being highly visible in transmitted light, it can significantly impact the overall color of the diamond. Indeed, it is not unusual for a diamond to fall an entire color grade after fracture-filling. For this reason fracture-filling is normally only applied to stones whose size is large enough to justify the treatment: however, stones as small as 0.02 carats (4 mg) have been fracture-filled.

The fracture-filling of diamond is a controversial treatment within the industry — and increasingly among the public as well — due to its radical and impermanent nature. The filling glass melts at such a low temperature (1,400 °C or 1,670 K) that it easily "sweats" out of a diamond under the heat of a jeweler's torch; thus routine jewelry repair can lead to a complete degradation of clarity or in some cases shattering, especially if the jeweler is not aware of the treatment. Similarly, a fracture-filled diamond placed in an ultrasonic cleaner may not survive intact.

It is notable that most major gemological laboratories, including that of the influential Gemological Institute of America, refuse to issue certificates for fracture-filled diamonds. Labs that do certify these diamonds may render any treatment benefit moot by disregarding apparent clarity and instead assigning the diamond a grade reflecting its original, pre-treatment clarity.

Diamond simulant From Wikipedia, the free encyclopedia

Jump to: navigation, search

Due to its low cost and close visual likeness to diamond, cubic zirconia has remained the most gemologically and economically important diamond simulant since 1976.

The high price of gem-grade diamonds, as well as significant ethical concerns of the diamond trade,[1] have created a large demand for materials with similar gemological characteristics, known as diamond simulants or imitations. Simulants are distinct from synthetic diamond, which is actual diamond having the same material properties as natural diamond. Enhanced diamonds are also excluded from this definition. A diamond simulant may be artificial, natural, or in some cases a combination thereof. While their material properties depart markedly from those of diamond, simulants have certain desired characteristics—such as dispersion and hardness—which lend themselves to imitation. Trained gemologists with appropriate equipment are able to distinguish natural and synthetic diamonds from all diamond simulants, primarily by visual inspection.

The most common diamond simulants are high-leaded glass (i.e., rhinestones) and cubic zirconia (CZ), both artificial materials. A number of other artificial materials, such as strontium titanate and synthetic rutile have been developed since the mid 1950s, but these are no longer in common use. Introduced at the end of the 20th century, the lab grown product moissanite has gained popularity as an alternative to diamond.

Contents • 1 Desired and differential properties

o 1.1 Durability and density o 1.2 Optics and color o 1.3 Thermal and electrical

• 2 Artificial simulants o 2.1 Summary table o 2.2 1700 onwards o 2.3 1900–1947 o 2.4 1947–1970 o 2.5 1970–1976 o 2.6 1976 to present

• 3 Natural simulants • 4 Composites • 5 See also • 6 Footnotes • 7 References

Desired and differential properties See also: Material properties of diamond

In order to be considered for use as a diamond simulant, a material must possess certain diamond-like properties. The most advanced artificial simulants have properties which closely approach diamond, but all simulants have one or more features that clearly and (for those familiar with diamond) easily differentiate them from diamond. To a gemologist, the most important of differential properties are those that foster non-destructive testing, and most of these are visual in nature. Non-destructive testing is preferred because most suspected diamonds are already cut into gemstones and set in jewelry, and if a destructive test (which mostly relies on the relative fragility and softness of non-diamonds) fails it may damage the simulant—this is not an acceptable outcome for most jewelry owners, as even if a stone is not a diamond it may still be of value.

Following are some of the properties by which diamond and its simulants can be compared and contrasted.

Durability and density

The Mohs scale of mineral hardness is a non-linear scale of common minerals' resistances to scratching. Diamond is at the top of this scale (hardness 10) as it is one of the hardest naturally-

occurring materials known. (Some artificial substances, such as aggregated diamond nanorods, are harder.) Since diamonds are unlikely to encounter substances that can scratch it, other than another diamond, diamond gemstones are typically free of scratches. Diamond's hardness also is visually evident (under the microscope or loupe) by its highly lustrous facets (described as adamantine) which are perfectly flat, and its crisp, sharp facet edges. For a diamond simulant to be effective, it must be very hard relative to most gems. Most simulants fall far short of diamond's hardness, so they can be separated from diamond by their external flaws and poor polish.

In the recent past, the so-called "window pane test" was commonly thought to be an assured method of identifying diamond. It is a potentially destructive test wherein a suspect diamond gemstone is scraped against a pane of glass, with a positive result being a scratch on the glass and none on the gemstone. The use of hardness points and scratch plates made of corundum (hardness 9) are also used in place of glass. Hardness tests are inadvisable for three reasons: glass is fairly soft (typically 6 or below) and can be scratched by a large number of materials (including many simulants); diamond has four directions of perfect and easy cleavage (planes of structural weakness along which the diamond could split) which could be triggered by the testing process; and many diamond-like gemstones (including older simulants) are valuable in their own right.

The specific gravity (SG) or density of a gem diamond is fairly constant at 3.52. Most simulants are far above or slightly below this value, which can make them easy to identify if unset. High-density liquids such as diiodomethane can be used for this purpose, but they are all highly toxic so are usually avoided. A more practical method is to compare the expected size and weight of a suspect diamond to its measured parameters: for example, a cubic zirconia (SG 5.6–6) will be 1.7 times the expected weight of an equivalently sized diamond.

Optics and color

Diamonds are usually cut into brilliants to bring out their brilliance, the amount of light reflected back to the viewer, and fire, the degree of colorful prismatic flashes seen. Both properties are strongly affected by the cut of the stone, but they are a function of diamond's high refractive index (RI; the degree to which incident light is bent upon entering the stone) of 2.417 (as measured by sodium light, 589.3 nm) and high dispersion (the degree to which white light is split into its spectral colors within the stone) of 0.044, as measured by the sodium B and G line interval. Thus, if a diamond simulant's RI and dispersion are too low it will appear comparatively dull or "lifeless"; if the RI and dispersion are too high, the effect will be considered unreal or even tacky. Very few simulants have closely approximating RI and dispersion, but even the close simulants can be separated by an experienced observer. Direct measurements of RI and dispersion are impractical (a standard gemological refractometer has an upper limit of about RI 1.81), but several companies have devised reflectivity meters to gauge a material's RI indirectly by measuring how well it reflects an infrared beam.

Perhaps equally as important is optic character. Diamond and other cubic (and also amorphous) materials are isotropic, meaning light entering a stone behaves the same way regardless of direction. Conversely, most minerals are anisotropic which produces birefringence or double refraction of light entering the material in all directions other than an optic axis (a direction of single refraction in a doubly refractive material). Under low magnification, this birefringence is usually detectable as a visual doubling of a cut gemstone's rear facets or internal flaws. An effective diamond simulant should therefore be isotropic.

Under longwave (365 nm) ultraviolet light, diamond may fluoresce a blue, yellow, green, mauve, or red of varying intensity. The most common fluorescence is blue, and such stones may also phosphoresce yellow—this is thought to be a unique combination among gemstones. There is usually little if any response to shortwave ultraviolet, in contrast to many diamond simulants. Similarly, because most diamond simulants are artificial they tend to have uniform properties: in a multi-stone diamond ring, one would expect the individual diamonds to fluoresce differently (in different colors and intensities, with some likely to be inert). If all the stones fluoresce in an identical manner, they are unlikely to be diamond.

Most "colorless" diamonds are actually tinted yellow or brown to some degree, whereas some artificial simulants are completely colorless—the equivalent of a perfect "D" in diamond color terminology. This "too good to be true" factor is important to consider; colored diamond simulants meant to imitate fancy diamonds are more difficult to spot in this regard, but the simulants' colors rarely approximate. In most diamonds (even colorless ones) a characteristic absorption spectrum can be seen (via a direct-vision spectroscope), consisting of a fine line at 415 nm. The dopants used to impart color in artificial simulants may be detectable as a complex rare earth absorption spectrum, which is never seen in diamond.

Also present in most diamonds are certain internal and external flaws or inclusions, the most common of which are fractures and solid foreign crystals. Artificial simulants are usually internally flawless, and any flaws that are present are characteristic of the manufacturing process. The inclusions seen in natural simulants will often be unlike those ever seen in diamond, most notably liquid "feather" inclusions. The diamond cutting process will often leave portions of the original crystal's surface intact. These are termed naturals and are usually on the girdle of the stone; they take the form of triangular, rectangular, or square pits (etch marks) and are seen only in diamond.

Thermal and electrical

Diamond is an extremely effective thermal conductor and usually an electrical insulator. The former property is widely exploited in the use of an electronic thermal probe to separate diamonds from their imitations. These probes consist of a pair of battery-powered thermistors mounted in a fine copper tip. One thermistor functions as a heating device while the other measures the temperature of the copper tip: if the stone being tested is a diamond, it will conduct the tip's thermal energy rapidly enough to produce a measurable temperature drop. As most simulants are thermal insulators, the thermistor's heat will not be conducted. This test takes about 2–3 seconds. The only possible exception is moissanite, which has a thermal conductivity similar to diamond: older probes can be fooled by moissanite, but newer Thermal and Electrical Conductivity testers are sophisticated enough to differentiate the two materials. The latest development is nano diamond coating, an extremely thin layer of diamond material. If not tested properly it may show the same characteristics as a diamond.[2]

A diamond's electrical conductance is only relevant to blue or gray-blue stones, because the interstitial boron responsible for their color also makes them semiconductors. Thus a suspected blue diamond can be affirmed if it completes an electric circuit successfully.

Artificial simulants

Diamond has been imitated by artificial materials for hundreds of years: advances in technology have seen the development of increasingly better simulants with properties ever nearer those of diamond. Although most of these simulants were characteristic of a certain time period, their large production volumes ensured that all continue to be encountered with varying frequency in jewelry of the present. Nearly all were first conceived for intended use in high technology, such as active laser mediums, varistors, and bubble memory. Due to their limited present supply, collectors may pay a premium for the older types.

Summary table

Diamond simulants and their gemological properties

Material Formula Refractive index(es) 589.3 nm

Dispersion 431 –

687 nm

Hardness (Mohs' scale)

Density (g/cm3)

Thermal Cond.

State of the art

Diamond C 2.417 0.044 10 3.52 Excellent 1476 –

Artificial Simulants:

Glasses

Silica with Pb, Al, &/or Tl

~ 1.6 > 0.020 < 6 2.4 – 4.2 Poor 1700 –

White Sapphire Al2O3 1.762 – 1.770

0.018 9 3.97 Poor 1900–1947

Spinel MgO·Al2O3 1.727 0.020 8 ~ 3.6 Poor 1920–1947

Rutile TiO2 2.62 – 2.9 0.33 ~ 6 4.25 Poor 1947–1955

Strontium titanate

SrTiO3 2.41 0.19 5.5 5.13 Poor 1955–1970

YAG Y3Al5O12 1.83 0.028 8.25 4.55 – 4.65

Poor 1970–1975

GGG Gd3Ga5O12 1.97 0.045 7 7.02 Poor 1973–1975

Cubic Zirconia ZrO2(+ rare earths) ~ 2.2 ~ 0.06 ~ 8.3 ~ 5.7 Poor 1976 –

Moissanite SiC 2.648 – 2.691

0.104 8.5–9.25 3.2 High 1998 –

Natural Simulants:

Quartz Silica

1.543 – 1.554

7- 2.50 – 2.65

Ancient

The "refractive index(es)" column shows one refractive index for singly refractive substances, and a range for doubly refractive substances.

1700 onwards

The formulation of glasses using lead, alumina, and thallium to increase RI and dispersion began in the late Baroque period. These glasses are fashioned into brilliants, and when freshly cut they can be surprisingly effective diamond simulants. Known as rhinestones, pastes, or strass, glass simulants are a common feature of antique jewelry, and in such cases rhinestones can be valuable historical artifacts in their own right. The great softness (below hardnes 6) imparted by the lead means a rhinestone's facet edges and faces will quickly become rounded and scratched. Together with conchoidal fractures, and air bubbles or flow lines within the stone, these features make glass imitations easy to spot under only moderate magnification. In contemporary production it is more common for glass to be molded rather than cut into shape: in these stones the facets will be concave and facet edges rounded, and mold marks or seams may also be present. Glass has also been combined with other materials to produce composites.

1900–1947

The first crystalline artificial diamond simulants were synthetic white sapphire (Al2O3, pure corundum) and spinel (MgO·Al2O3, pure magnesium aluminium oxide). Both have been synthesized in large quantities since the first decade of the 20th century via the Verneuil or flame-fusion process, although spinel was not in wide use until the 1920s. The Verneuil process involves an inverted oxyhydrogen blowpipe, with purified feed powder mixed with oxygen that is carefully fed through the blowpipe. The feed powder falls through the oxy-hydrogen flame, melts, and lands on a rotating and slowly descending pedestal below. The height of the pedestal is constantly adjusted to keep its top at the optimal position below the flame, and over a number of hours the molten powder cools and crystallizes to form a single pedunculated pear or boule crystal. The process is an economical one, with crystals of up to 9 centimeters (3.5 inches) in diameter grown. Boules grown via the modern Czochralski process may weigh several kilograms.

Synthetic sapphire and spinel are durable materials (hardness 9 and 8) that take a good polish, but due to their much lower RI when compared to diamond (1.762–1.770 for sapphire, 1.727 for spinel) they are "lifeless" when cut. (Synthetic sapphire is also anisotropic, making it even easier to spot.) Their low RIs also mean a much lower dispersion (0.018 and 0.020), so even when cut into brilliants they lack the fire of diamond. Nevertheless synthetic spinel and sapphire were popular diamond simulants from the 1920s up until the late 1940s, when newer and better simulants began to appear. Both have also been combined with other materials to create composites. Commercial names once used for synthetic sapphire include Diamondette, Diamondite, Jourado Diamond', and Thrilliant. Names for synthetic spinel included Corundolite, Lustergem, Magalux, and Radiant.

1947–1970

The first of the optically "improved" simulants was synthetic rutile (TiO2, pure titanium oxide). Introduced in 1947–48, synthetic rutile possesses plenty of life when cut—perhaps too much life for a diamond simulant. Synthetic rutile's RI and dispersion (2.8 and 0.33) are so much higher than diamond that the resultant brilliants look almost opal-like in their display of prismatic colors. Synthetic rutile is also doubly refractive: although some stones are cut with the table perpendicular to the optic axis to hide this property, merely tilting the stone will reveal the doubled back facets.

The continued success of synthetic rutile was also hampered by the material's inescapable yellow tint, which producers were never able to remedy. However, synthetic rutile in a range of different colors, including blues and reds, were produced using various metal oxide dopants. These and the near-white stones were extremely popular if unreal stones. Synthetic rutile is also fairly soft (hardness ~6) and brittle, and therefore wears poorly. It is synthesized via a modification of the Verneuil process, which uses a third oxygen pipe to create a tricone burner—this is necessary to produce a single crystal, due to the much higher oxygen losses involved in the oxidation of titanium. The technique was invented by Charles H. Moore, Jr. at the South Amboy, New Jersey-based National Lead Company (later NL Industries). National Lead and Union Carbide were the primary producers of synthetic rutile, and peak annual production reached 750,000 carats (150 kg). Some of the many commercial names applied to synthetic rutile include: Astryl, Diamothyst, Gava or Java Gem, Meredith, Miridis, Rainbow Diamond, Rainbow Magic Diamond, Rutania, Titangem, Titania, and Ultamite.

National Lead was also where research into the synthesis of another titanium compound, strontium titanate (SrTiO3, pure tausonite), was conducted. Research was done during the late 1940s and early 1950s by Leon Merker and Langtry E. Lynd, who also used a tricone modification of the Verneuil process. Upon its commercial introduction in 1955, strontium titanate quickly replaced synthetic rutile as the most popular diamond simulant. This was due not only to strontium titanate's novelty, but to its superior optics: its RI (2.41) is very close to that of diamond, while its dispersion (0.19), although also very high, was a significant improvement over synthetic rutile's psychedelic display. Dopants were also used to give synthetic titanate a variety of colors, including yellow, orange to red, blue, and black. The material is also isotropic like diamond, meaning there is no distracting doubling of facets as seen in synthetic rutile.

Strontium titanate's only major drawback (if one excludes excess fire) is fragility. It is both softer (hardness 5.5) and more brittle than synthetic rutile—for this reason, strontium titanate was also combined with more durable materials to create composites. It was otherwise the best simulant around at the time, and at its peak annual production was 1.5 million carats (300 kg). Due to patent coverage all US production was by National Lead, while large amounts were produced overseas by Nakazumi Company of Japan. Commercial names for strontium titanate included Brilliante, Diagem, Diamontina, Fabulite, and Marvelite.

1970–1976

From about 1970 strontium titanate began to be replaced by a new class of diamond imitations: the "synthetic garnets." These are not true garnets in the usual sense because they are oxides rather than silicates, but they do share natural garnet's crystal structure (both are cubic and therefore isotropic) and the general formula A3B2C3O12. While in natural garnets C is always silicon and A and B may be one of several common elements, most synthetic garnets are composed of uncommon rare earth elements. They are the only diamond simulants (aside from rhinestones) with no known

natural counterparts: gemologically they are best termed artificial rather than synthetic, because the latter term is reserved for human-made materials that can also be found in nature.

Although a number of artificial garnets were successfully grown, only two became important as diamond simulants. The first was yttrium aluminium garnet (YAG; Y3Al5O12) in the late 1960s. It was (and still is) produced via the Czochralski or crystal-pulling process, which involves growth from the melt. An iridium crucible surrounded by an inert atmosphere is used, wherein yttrium oxide and aluminium oxide are melted and mixed together at a carefully controlled temperature of ca. 1980 °C. A small seed crystal is attached to a rod which is lowered over the crucible until the crystal contacts the surface of the melted mixture. The seed crystal acts as a site of nucleation; the temperature is kept steady at a point where the surface of the mixture is just below the melting point. The rod is slowly and continuously rotated and retracted, and the pulled mixture crystallizes as it exits the crucible, forming a single crystal in the form of a cylindrical boule. The crystal's purity is extremely high, and it typically measures 5 cm (2 inches) in diameter and 20 cm (8 inches) long, and weighs 9,000 carats (1.75 kg).

YAG's hardness (8.25) and lack of brittleness were great improvements over strontium titanate, and although its RI (1.83) and dispersion (0.028) were fairly low, they were enough to give brilliant-cut YAGs perceptible fire and good brilliance (although still much lower than diamond). A number of different colors were also produced with the addition of dopants, including yellow, red, and a vivid green which was used to imitate emerald. Major producers included ICT, INC. of Michigan, Litton Systems, Allied Chemical, Raytheon, and Union Carbide; annual global production peaked at 40 million carats (8,000 kg) in 1972, but fell sharply thereafter. Commercial names for YAG included Diamonair, Diamonique, Gemonair, Replique, and Triamond.

While market saturation was one reason for the fall in YAG production levels, another was the recent introduction of the other artificial garnet important as a diamond simulant, gadolinium gallium garnet (GGG; Gd3Ga5O12). Produced in much the same manner as YAG (but with a lower melting point of 1750 °C), GGG had an RI (1.97) close to, and a dispersion (0.045) nearly identical to diamond. GGG was also hard enough (hardness 7) and tough enough to be an effective gemstone, but its ingredients were also much more expensive than YAG's. Equally hindering was GGG's tendency to turn a dark brown upon exposure to sunlight or other ultraviolet source: this was due to the fact that most GGG gems were fashioned from impure material that was rejected for technological use. The SG of GGG (7.02) is also the highest of all diamond simulants and amongst the highest of all gemstones, which makes loose GGG gems easy to spot by comparing their dimensions with their expected and actual weights. Relative to its predecessors, GGG was never produced in significant quantities; it became more or less unheard of by the close of the 1970s. Commercial names for GGG included Diamonique II and Galliant.

1976 to present

Cubic zirconia or CZ (ZrO2; zirconium dioxide—not to be confused with zircon, a zirconium silicate) quickly dominated the diamond simulant market following its introduction in 1976, and it remains the most gemologically and economically important simulant. CZ had been synthesized since 1930 but only in ceramic form: the growth of single-crystal CZ would require an approach radically different from those used for previous simulants due to zirconium's extremely high melting point (2750 °C), unsustainable by any crucible. The solution found involved a network of water-filled copper pipes and radio-frequency induction heating coils; the latter to heat the zirconium feed

powder, and the former to cool the exterior and maintain a retaining "skin" under 1 millimeter thick. CZ was thus grown in a crucible of itself, a technique called cold crucible (in reference to the cooling pipes) or skull crucible (in reference to either the shape of the crucible or of the crystals grown).

At standard pressure zirconium oxide would normally crystallize in the monoclinic rather than cubic crystal system: for cubic crystals to grow, a stabilizer must be used. This is usually Yttrium(III) oxide or calcium oxide. The skull crucible technique was first developed in 1960s France, but was perfected in the early 1970s by Soviet scientists under V. V. Osiko at the Lebedev Physical Institute in Moscow. By 1980 annual global production had reached 50 million carats (10,000 kg).

The hardness (8–8.5), RI (2.15–2.18, isotropic), dispersion (0.058–0.066), and low material cost make CZ the most popular simulant of diamond. Its optical and physical constants are however variable, owing to the different stabilizers used by different producers. There are many formulations of stabilized cubic zirconia. These variations change the physical and optical properties markedly. While the visual likeness of CZ is close enough to diamond to fool most who do not handle diamond regularly, CZ will usually give certain clues. For example: it is somewhat brittle and is soft enough to possess scratches after normal use in jewelry; it is usually internally flawless and completely colorless (whereas most diamonds have some internal imperfections and a yellow tint); its SG (5.6–6) is high; and its reaction under ultraviolet light is a distinctive beige. Most jewelers will use a thermal probe to test all suspected CZs, a test which relies on diamond's superlative thermal conductivity (CZ, like almost all other diamond simulants, is a thermal insulator). CZ is made in a number of different colors meant to imitate fancy diamonds (e.g., yellow to golden brown, orange, red to pink, green, and opaque black), but most of these do not approximate the real thing. Cubic zirconia can be coated with diamond-like carbon to improve its durability, but will still be detected as CZ by a thermal probe.

CZ had virtually no competition until the 1998 introduction of moissanite (SiC; silicon carbide). Moissanite is superior to cubic zirconia in two ways: its hardness (8.5–9.25) and low SG (3.2). The former property results in facets that are sometimes as crisp as a diamond's, while the latter property makes simulated moissanite somewhat harder to spot when unset (although still disparate enough to detect). However, unlike diamond and cubic zirconia, moissanite is strongly birefringent. This manifests as the same "drunken vision" effect seen in synthetic rutile, although to a lesser degree. All moissanite is cut with the table perpendicular to the optic axis in order to hide this property from above, but when viewed under magnification at only a slight tilt the doubling of facets (and any inclusions) is readily apparent.

The inclusions seen in moissanite are also characteristic: most will have fine, white, subparallel growth tubes or needles oriented perpendicular to the stone's table. It is conceivable that these growth tubes could be mistaken for laser drill holes that are sometimes seen in diamond (see diamond enhancement), but the tubes will be noticeably doubled in moissanite due to its birefringence. Like synthetic rutile, current moissanite production is also plagued by an as of yet inescapable tint, which is usually a brownish green. A limited range of fancy colors have been produced as well, the two most common being blue and green. Jewel-quality moissanite is produced by only one company, Charles & Colvard. Its limited availability makes moissanite about 120 times more expensive than cubic zirconia.

Natural simulants

Natural minerals that (when cut) optically resemble white diamonds are rare, because the trace impurities usually present in natural minerals tend to impart color. The earliest simulants of diamond were colorless quartz (A form of silica, which also form obsidian, glass and sand), crystal (a type of quartz), topaz, and beryl (goshenite); they are all common minerals with above-average hardness (7–8), but all have low RIs and correspondingly low dispersions. Well-formed quartz crystals are sometimes offered as "diamonds," a popular example being the so-called "Herkimer diamonds" mined in Herkimer County, New York. Topaz's SG (3.50–3.57) also falls within the range of diamond.

From a historical perspective, the most notable natural simulant of diamond is zircon. It is also fairly hard (7.5), but more importantly shows perceptible fire when cut, due to its high dispersion of 0.039. Colorless zircon has been mined in Sri Lanka for over 2,000 years; prior to the advent of modern mineralogy, colorless zircon was thought to be an inferior form of diamond. It was called "Matara diamond" after its source location. It is still encountered as a diamond simulant, but differentiation is easy due to zircon's anisotropy and strong birefringence (0.059). It is also notoriously brittle and often shows wear on the girdle and facet edges.

Much less common than colorless zircon is colorless scheelite. Its dispersion (0.026) is also high enough to mimic diamond, but although it is highly lustrous its hardness is much too low (4.5–5.5) to maintain a good polish. It is also anisotropic and fairly dense (SG 5.9–6.1). Synthetic scheelite produced via the Czochralski process is available, but it has never been widely used as a diamond simulant. Due to the scarcity of natural gem-quality scheelite, synthetic scheelite is much more likely to simulate it than diamond. A similar case is the orthorhombic carbonate cerussite, which is so fragile (very brittle with four directions of good cleavage) and soft (hardness 3.5) that it is never seen set in jewelry, and only occasionally seen in gem collections because it is so difficult to cut. Cerussite gems have an adamantine luster, high RI (1.804–2.078), and high dispersion (0.051), making them attractive and valued collector's pieces. Aside from softness, they are easily distinguished by cerussite's high density (SG 6.51) and anisotropy with extreme birefringence (0.271).

Due to their rarity fancy-colored diamonds are also imitated, and zircon can serve this purpose too. Applying heat treatment to brown zircon can create several bright colors: these are most commonly sky-blue, golden yellow, and red. Blue zircon is very popular, but it is not necessarily color stable; prolonged exposure to ultraviolet light (including the UV component in sunlight) tends to bleach the stone. Heat treatment also imparts greater brittleness to zircon and characteristic inclusions.

Another fragile candidate mineral is sphalerite (zinc blende). Gem-quality material is usually a strong yellow to honey brown, orange, red, or green; its very high RI (2.37) and dispersion (0.156) make for an extremely lustrous and fiery gem, and it is also isotropic. But here again, its low hardness (2.5–4) and perfect dodecahedral cleavage preclude sphalerite's wide use in jewelry. Two calcium-rich members of the garnet group fare much better: these are grossularite (usually brownish orange, rarely colorless, yellow, green, or pink) and andradite. The latter is the rarest and most costly of the garnets, with three of its varieties—topazolite (yellow), melanite (black), and demantoid (green)—sometimes seen in jewelry. Demantoid (literally "diamond-like") especially has been prized as a gemstone since its discovery in the Ural Mountains in 1868; it is a noted feature of antique Russian and Art Nouveau jewelry. Titanite or sphene is also seen in antique jewelry; it is typically some shade of chartreuse and has a luster, RI (1.885–2.050), and dispersion (0.051) high enough to be mistaken for diamond, yet it is anisotropic (a high birefringence of 0.105–0.135) and soft (hardness 5.5).

Discovered the 1960s, the rich green tsavorite variety of grossular is also very popular. Both grossular and andradite are isotropic and have relatively high RIs (ca. 1.74 and 1.89, respectively) and high dispersions (0.027 and 0.057), with demantoid's exceeding diamond. However, both have a low hardness (6.5–7.5) and invariably possess inclusions atypical of diamond—the byssolite "horsetails" seen in demantoid are one striking example. Furthermore, most are very small, typically under 0.5 carats (100 mg) in weight. Their lusters range from vitreous to subadamantine, to almost metallic in the usually opaque melanite, which has been used to simulate black diamond. Some natural spinel is also a deep black and could serve this same purpose.

Composites Because strontium titanate and glass are too soft to survive use as a ring stone, they have been used in the construction of composite or doublet diamond simulants. The two materials are used for the bottom portion (pavilion) of the stone, and in the case of strontium titanate, a much harder material—usually colorless synthetic spinel or sapphire—is used for the top half (crown). In glass doublets, the top portion is made of almandine garnet; it is usually a very thin slice which does not modify the stone's overall body color. There have even been reports of diamond-on-diamond doublets, where a creative entrepreneur has used two small pieces of rough to create one larger stone.

In strontium titanate and diamond-based doublets, an epoxy is used to adhere the two halves together. The epoxy may fluoresce under UV light, and there may be residue on the stone's exterior. The garnet top of a glass doublet is physically fused to its base, but in it and the other doublet types there are usually flattened air bubbles seen at the junction of the two halves. A join line is also readily visible whose position is variable; it may be above or below the girdle, sometimes at an angle, but rarely along the girdle itself.

The most recent composite simulant involves combining a CZ core with an outer coating of laboratory created amorphous diamond. The concept effectively mimics the structure of a cultured pearl (which combines a core bead with an outer layer of pearl coating), only done for the diamond market.

Diamond Glossary

American Gem Society (AGS): An educational institution for gemological studies. The AGS Labs were created primarily to develop and promote universally-accepted standards for grading cut.

Blemish: A clarity characteristic that occurs on the surface of a diamond. Though some blemishes are inherent to the original rough diamond, most are the result of the environment the diamond has encountered since it was unearthed.

Brilliance: The brightness that seems to come from the very heart of a diamond. It is the effect that makes diamonds unique among all other gemstones. While other gemstones also display brilliance, none have the power to equal the extent of diamond's light-reflecting power. Brilliance is created primarily when light enters through the table, reaches the pavilion facets, and is then reflected back out through the table, where the light is most visible to your eye.

Brilliant Cut: One of three styles of faceting arrangements. In this type of arrangement, all facets appear to radiate out from the center of the diamond toward its outer edges. It is called a brilliant cut because it is designed to maximize brilliance. Round diamonds, ovals, radiants, princesses, hearts, marquises, and pears all fall within this category of cut.

Carat: The unit of weight by which a diamond is measured. One carat equals 200 milligrams, or 0.2 grams. The word comes from the carob bean, whose consistent weight was used in times past to measure gemstones.

Carbon Spots: An inaccurate term used by some people in the jewelry industry to describe the appearance of certain inclusions in a diamond. The term refers to included crystals that have a dark appearance, rather than a white or transparent appearance, when viewed under a microscope. In most cases, these dark inclusions are not visible to the naked eye, and do not affect the brilliance of the diamond.

Cleavage: The propensity of crystalline minerals, such as diamond, to split in one or more directions either along or parallel to certain planes, when struck by a blow. Cleavage is one of the two methods used by diamond cutters to split rough diamond crystals in preparation for the cutting process (sawing is the other method).

Clouds: A grouping of a number of extremely tiny inclusions that are too small to be distinguishable from one another, even under magnification. The result is that, under a microscope, this grouping often looks like a soft transparent cloud inside the diamond. Of course, clouds cannot be seen with the naked eye. Usually, this sort of inclusion does not significantly impact a diamond's clarity grade.

Color Grading: A system of grading diamond colors based on their colorlessness (for white diamonds) or their spectral hue, depth of color and purity of color (for fancy color diamonds). For white diamonds, GIA and AGS use a grading system which runs from D (totally colorless) to Z (light yellow).

Crown: The upper portion of a cut gemstone, which lies above the girdle. The crown consists of a table facet surrounded by either star and bezel facets (on round diamonds and most fancy cuts) or concentric rows of facets reaching from the table to the girdle (on emerald cuts and other step cuts).

Crown angle: The angle at which a diamond's bezel facets (or, on emerald cuts, the row of concentric facets) intersect the girdle plane. This gentle slope of the facets that surround the table is what helps to create the dispersion, or fire, in a diamond. White light entering at the different angles in broken up into its spectral hues, creating a beautiful play of color inside the diamond. The crown angle also helps to enhance the brilliance of a diamond.

Culet: A tiny flat facet that diamond cutters sometimes add at the bottom of a diamond's pavilion. Its purpose is to protect the tip of the pavilion from being chipped or damaged. Once a diamond is set in jewelry, though, the setting itself generally provides the pavilion with sufficient protection from impact or wear. Large or extremely large culets were common in diamonds cut in the early part of this century, such as the Old European or Old Mine Cut. However, such large culets are rarely seen today. Most modern shapes have either no culet at all, or a small or very small culet.

Cut: This refers both to the proportions and finish of a polished diamond. As one of "the Four Cs" of diamond value, it is the only man-made contribution to a diamond's beauty and value.

Depth: The height of a diamond from the culet to the table. The depth is measured in millimeters.

Depth Percentage: On a diamond grading report, you will see two different measurements of the diamond's depth-the actual depth in millimeters (under "measurements" at the top of the report) and the depth percentage, which expresses how deep the diamond is in comparison to how wide it is. This depth percentage of a diamond is important to its brilliance and value, but it only tells part of the story. Where that depth lies is equally important to the diamond's beauty; specifically, the pavilion should be just deep enough to allow light to bounce around inside the diamond and be reflecting out to the eye at the proper angle. Keep in mind, also, that a depth percentage that might be excessive for one diamond cut might be necessary for another type of cut. For example, a 75% or 78% depth in a princess cut diamond would be typical and quite attractive. However, a depth of even 65% would be unnecessary and even detrimental to a round diamond's beauty.

Diamond: A crystal made up of 99.95% pure carbon atoms arranged in an isometric, or cubic, crystal arrangement. It is this unique arrangement of the carbon atoms that makes diamond look and behave differently from other pure carbon minerals such as graphite (the soft black material used to make pencils).

Diamond Cutting: The method by which a rough diamond that has been mined from the earth is shaped into a finished, faceted stone. As a first step, cleaving or sawing is often used to separate the rough into smaller, more workable pieces that will each eventually become an individual polished gem. Next, bruting grinds away the edges, providing the outline shape (for example, heart, oval or round) for the gem. Faceting is done in two steps: during blocking, the table, culet, bezel and pavilion main facets are cut; afterward, the star, upper girdle and lower girdle facets are added. Once the fully faceted diamond has been inspected and improved, it is boiled in hydrochloric and sulfuric acids to remove dust and oil. The diamond is then considered a finished, polished gem.

Diamond Gauge: An instrument that is used to measure a diamond's length, width and depth in millimeters.

Dispersion: Arranged around the table facet on the crown are several smaller facets (bezel and star facets) angled downward at varying degrees. These facets, and the angles at which they are cut, have been skillfully designed to break up white light as it hits the surface, separating it into its component spectral colors (for example, red, blue and green). This effect, which appears as a play of small flashes of color across the surface of the diamond as it is tilted, is what we refer to as the diamond's dispersion (also called "fire"). This play of color should not be confused with a diamond's natural body color (normally white, though sometimes yellow, brown, pink or blue in the case of fancy color diamonds) which is uniform throughout the entire diamond and is constant, regardless of whether it is being tilted or not.

Emerald Cut: A square or rectangular-shaped diamond with cut corners. On the crown, there are three concentric rows of facets arranged around the table and, on the pavilion, there are

three concentric rows arranged around the culet. This type of cut is also known as a Step Cut because its broad, flat planes resemble stair steps.

Eye-Clean: An term used in the jewelry industry to describe a diamond with no blemishes or inclusions that are visible to the naked eye (i.e. a human eye which is not aided by magnifying devices such as a jeweler's loupe or a microscope).

Facet: The smooth, flat faces on the surface of a diamond. They allow light to both enter a diamond and reflect off its surface at different angles, creating the wonderful play of color and light for which diamonds are famous. The table below shows all the facets on a round brilliant cut diamond. A round brilliant has 58 facets (or 57 if there is no culet). The shape, quantity, and arrangement of these facets will differ slightly among other fancy shapes.

Fancy Shape: Any diamond shape other than round.

Feathers: These are small fractures in a diamond. They are usually caused by the tremendous stress that the diamond suffered while it was growing underground. In some cases the feather both begins and ends within the diamond's surface and, in other cases, the feather begins inside the diamond and extends to the surface. When viewed under magnification, some feathers are transparent and others have a light white appearance to them. The term "feather" comes from the fact that, under magnification, these fractures often seem to have an indistinct, feathery shape to them. While the idea of buying a diamond with "fractures" may sound scary, the reality is that, with normal wear and care, most feathers pose no risk to the diamond's stability. Consider this: even with the feathers, these diamonds survived their growth and their journey to the surface intact. Once on the surface, they also survived the mining process, as well as the brutal stresses of the diamond cutting process. Though diamonds are certainly not invulnerable to damage, basic consideration to their care and handling during everyday wear will most likely protect them over the course of several human lifetimes.

Finish: This term refers to the qualities imparted to a diamond by the skill of the diamond cutter. The term "finish" covers every aspect of a diamond's appearance that is not a result of the diamond's inherent nature when it comes out of the ground. The execution of the diamond's design, the precision of its cutting details, and the quality of its polish are all a

consideration when a gemologist is grading finish. If you examine a diamond's grading report, you will see its finish graded according to two separate categories: polish and symmetry.

Fire: See "dispersion".

Fluorescence: An effect that is seen in some gem-quality diamonds when they are exposed to long-wave ultraviolet light (such as the lighting frequently seen in dance clubs). Under most lighting conditions, this fluorescence is not detectable to the eye. However, if a diamond is naturally fluorescent, it will emit a soft colored glow when held under an ultraviolet lamp or "black light." Fluorescence is not dangerous to the diamond or to the wearer; it is a unique and fascinating quality that occurs naturally in a number of gems and minerals.

Gemological Institute of America (GIA): Founded in 1931 by Roger Shipley, this non- profit organization upholds the highest standards for grading diamonds and other precious gems. The GIA has one of the most-respected and well-regarded gemological laboratories in the world; GIA was responsible for developing and standardizing the diamond grading system that is used today by nearly all other gem labs.

Girdle: The outer edge, or outline, of the diamond's shape. The girdle is not graded, but rather it is described by its appearance at its thinnest and thickest points. The descriptions of girdle thickness range as follows: extremely thin; thin; medium; slightly thick; thick; extremely thick. While it is less desirable for a round diamond to display an extremely thin or extremely thick girdle, such girdle widths are more common and acceptable in fancy shapes.

For example, shapes such as pears, marquises or hearts may be cut with extremely thick girdles at their points (and at the cleft, in the case of a heart) in order to protect these delicates corners from damage. Most diamonds have smooth girdles that are fashioned by a "bruter" (a diamond cutter who is responsible for shaping the diamond's basic outline) early on in the cutting process. In some cases, cutters go a step further and do additional cutting on the girdle. In these cases, they may decide to create a "polished" girdle or a "faceted" girdle. In both cases, the difference between these and a regular, smooth girdle is generally not distinguishable to the eye. A polished or faceted girdle doesn't improve a diamond's grade. Most labs grade a girdle's thickness, not its appearance.

Heart-shape Cut: A type of fancy diamond cut, which is cut to resemble the popular Valentine's Day shape.

Inclusion: A clarity characteristic found within a diamond. Most inclusions were created when the gem first formed in the earth.

Laser-Drill Holes: One of the few man-made inclusions that can occur inside a diamond. Why on earth would anyone want to drill holes into a perfectly good diamond? It may seem counter-intuitive, but drilling this type of hole into a diamond can actually raise its clarity grade. In some diamonds, the clarity grade may be determined mainly by the presence of just one or two dark included crystals in a diamond that is otherwise relatively free of inclusions. In certain circumstances, the diamond cutter will decide to use a procedure to remove the dark inclusions and, hopefully, increase the clarity of the diamond. First, a hole is precisely made with state-of-the-art equipment; it extends no further than it needs to, and its width is so small (about the size of a pinpoint) that a loupe or microscope is usually required to detect it. Next, a strong acid solution is forced into the new hole.

Since diamonds are resistant to acids, the solution actually dissolves the included crystal while leaving the diamond completely unharmed. The end result is a more transparent diamond. The

structural stability of the diamond is not compromised in any way by this hole, and the process is permanent.

Length-to-width ratio: A comparison of how much longer a diamond is than it is wide. It is used to analyze the outline of fancy shapes only; it is never applied to round diamonds. There's really no such thing as an 'ideal' ratio; it's simply a matter of personal aesthetic preferences. For example, while many people are told that a 2 to 1 ratio is best for a marquise, most people actually tend to prefer a ratio of around 1.80 to 1 when they actually look at marquises. And though the standard accepted range for the length-to- width ratio of a marquise generally falls between 1.70 to 1 and 2.05 to 1, there are customers who insist on having 'fatter' marquises of about 1.60 to 1 and other customers who want longer, thinner marquises of 2.25 to 1.

Marquise Cut: A type of fancy shape diamond which is elongated with points at each end.

Naturals: Small parts of the original rough diamond's surface which are left on the polished diamond, frequently on or near the girdle. While these are blemishes, they might also be regarded as a sign of skilled cutting; the presence of a natural reflects the cutter's ability to design a beautiful polished gem, while still retaining as much of the original crystal's weight as possible. In many cases, naturals do not affect the clarity grade. In most cases, they are undetectable to the naked eye.

Another type of natural is the Indented Natural; in this case, the portion of the original rough diamond's surface which is left on the polished diamond dips slightly inward, creating an indentation. Usually, the cutter makes an effort to cut the polished diamond so that the indented natural will be confined to either the girdle or the pavilion (making it undetectable to the naked eye in the face-up position).

Oval Cut: A type of fancy shape diamond which is essentially an elongated version of a round cut.

Pavé: A style of jewelry setting in which numerous small diamonds are mounted close together to create a glistening diamond crust that covers the whole piece of jewelry and obscures the metal under it.

Pavilion: The lower portion of the diamond, below the girdle.

Pear Cut: A type of fancy shape diamond that resembles a teardrop.

Point: A unit of measurement used to describe the weight of diamonds. One point is equivalent to one-hundredth of a carat.

Polish: Refers to any blemishes on the surface of the diamond which are not significant enough to affect the clarity grade of the diamond. Examples of blemishes that might be considered as 'polish' characteristics are faint polishing lines and small surface nicks or scratches. Polish is regarded as an indicator of the quality of as diamond's cut; it is graded as either Ideal, Excellent, Very Good, Good, Fair or Poor.

Princess Cut: A type of brilliant cut fancy shape that can be either square or rectangular.

Radiant Cut: A type of brilliant cut fancy shape that resembles a square or rectangle with the corners cut off.

Ratio: A comparison of how much longer a diamond is than it is wide. It is used to analyze the outline of fancy shapes only; it is never applied to round diamonds. There's really no such thing as an 'ideal' ratio; it's simply a matter of personal aesthetic preferences. For example, while many people are told that a 2 to 1 ratio is best for a marquise, most people actually tend to prefer a ratio of around 1.80 to 1 when they actually look at marquises. And though the standard accepted range for the length-to-width ratio of a marquise generally falls between 1.70 to 1 and 2.05 to 1, there are customers who insist on having 'fatter' marquises of about 1.60 to 1 and other customers who want longer, thinner marquises of 2.25 to 1.

Semi-mount: A jewelry setting that has the side stones already mounted, but which contains an empty set of prongs which are intended to mount a diamond center stone that the customer selects separately.

Single-cut: A very small round diamond with only 16 or 17 facets, instead of the normal 57 or 58 facets of a full cut round brilliant. Single cuts are occasionally used for pavé jewelry and other jewelry that utilizes numerous small diamonds set closely together.

Step Cut: One of three styles of faceting arrangements. In this type of arrangement (named because its broad, flat planes resemble stair steps), there are three concentric rows of facets arranged around the table and, on the pavilion, there are three concentric rows arranged around the culet. Other styles of faceting arrangements include the brilliant cut (in which all facets radiate out from the center of the diamond toward its outer edges) and the mixed cut (in which either the crown or pavilion of a diamond is cut as a brilliant cut, and the other part of the diamond is cut as a step cut).

Symmetry: Refers to variations in a diamond's symmetry. The small variations can include misalignment of facets or facets that fail to point correctly to the girdle (this misalignment is completely undetectable to the naked eye). Symmetry is regarded as an indicator of the quality of as diamond's cut; it is graded as either Ideal, Excellent, Very Good, Good, Fair or Poor.

Table: The flat facet on the top of the diamond. It is the largest facet on a cut diamond.

Table percentage: The value which represents how the diameter of the table facet compares to the diameter of the entire diamond. So, a diamond with a 60% table has a table which is 60% as wide as the diamond's outline. For a round diamond, gemologists calculate table percentage by dividing the diameter of the table, which is measured in millimeters (this

millimeter measurement does not appear on diamond grading reports) by the average girdle diameter (an average of the first two millimeter measurements on the top left-hand side of a diamond grading report). For a fancy shape diamond, table percentage is calculated by dividing the width of the table, at the widest part of the diamond, by the millimeter width of the entire stone (this total width measurement is the second of the three millimeter values in the top left-hand corner of the diamond grading report. Contrary to popular misconception, having a small table percentage (53% to 57%) does not make a round diamond any more brilliant than a diamond with a larger table.

Trilliant Cut: A type of brilliant fancy shape that is triangular.

Diamond flaws From Wikipedia, the free encyclopedia Jump to: navigation, search

Contents • 1 External flaws

o 1.1 Blemishes o 1.2 Scratches o 1.3 Extra facets o 1.4 Fracture o 1.5 Fingerprints o 1.6 Pits o 1.7 Nicks o 1.8 Naturals o 1.9 Carbons o 1.10 Chips

• 2 Internal flaws o 2.1 Crystal/mineral inclusions o 2.2 Pinpoint inclusions o 2.3 Needles o 2.4 Cloud o 2.5 Knots o 2.6 Grain lines o 2.7 Feathers o 2.8 Intergrowths o 2.9 Cleavage o 2.10 Bearding

• 3 See also • 4 References • 5 External links

Diamonds flaws are common and most diamonds are not perfect and most of them consist of some inclusion or imperfections. These inclusions are also known as flaws and exist in various forms, exterior and interior. Inclusions are also classified in the manner in which they were formed. For

example, syngenetic diamond inclusions are those inclusions which were formed while a diamond was being made. On the other hand, epigenetic inclusions were formed after a diamond was made.

Flaws The black and white photographs accompanying this chapter show some of the inclusions which normally occur in diamond. Most of these might appear black in normal lighting conditions. In trade, these are called as ‘carbon’ spot or may be cleavage cracks which have developed through uneven heating or a blow. Often these cleavage cracks appear to be dark because of light reflection. The presence or absence of flows or inclusions in a diamond is usually decided by examining the stone carefully with a lens having the standard magnifications of 10 x. No other magnification should be taken as authoritative. Stones which show no apparent flaws or inclusions under this magnification are regarded as flawless. The term for a scarcely perceptible inclusion is “V.V.S.” for very very slight (or small) – which is sometimes offered as flawless. Slightly larger flaws or groups of very thin inclusions are termed “V.S” (or very slight). Larger flaws or inclusions are termed as S.I. (or slightly included). Even larger flaws are termed as “1st pique”.

Slightly larger flaws classify the stone as “2nd pique” and so on. Any flaws which invisible with naked eye is sufficient to scale the price down considerably.

External flaws

Blemishes

These diamond flaws are present on the surface of a stone and can occur naturally. However, these are more likely to be caused due to the external environment, when a diamond was being cut and polished.

Scratches

These are fine lines found on the surface of the diamond. They may have been present naturally or caused when a diamond was cut. While minor scratches can be removed through proper polishing, deep scratches can rarely be removed by treating the diamond.

Extra facets

These are usually cut to remove blemishes or certain close to surface inclusions on diamonds. At times these extra facets are also cut to enhance the brilliance of the diamond. These do not usually affect the clarity grade of a diamond.

Fracture

A breakage in diamonds that is not parallel to the cleavage plane is referred to as a fracture. Fractures are usually irregular in shape making a diamond look chipped. The practice of fracture filling is commonly employed to improve the diamond clarity of such diamonds.

Fingerprints

Fingerprint inclusions in the shape of fingerprints can sometimes be found in diamonds. However such inclusions are rare in diamonds as compared to other stones like rubies. Such inclusions are usually formed during fluid assisted partial healing of fractures already present in stones. For this to take place in diamonds, high temperatures and pressure (HTHP) are required, which is unusual. Till now few such inclusions have been reported in natural blue and colorless diamonds. While this could indicate that diamonds have been HTHP treated, giving the required temperatures for fingerprint inclusions, such is not always the case. The earth may also cause geologically high temperatures, leading to the formation of fingerprint inclusions.

Pits

Small holes may be present on the surface of a diamond. These pits are usually not visible to the naked eye. However, pits present on the table facet of a diamond are usually visible and reduce the clarity of a diamond.

Nicks

Diamonds are also chipped at places causing the appearance of nicks. This is often repaired by adding extra facets. However too many facets reduce the brilliance of a diamond and are to be avoided.

Naturals

This refers to the original surface of the diamond which has not been polished and left as it is. Naturals are usually left on or near the girdle of the diamond. While these are considered as blemishes, the presence of naturals is a sign of good cutting practice, where the cutter has managed to retain as much of the original weight as possible. Indented naturals are also seen to exist on some stones, where the portion of the natural is seen to dip inside slightly from the diameter of the stone. Here the cutter usually leaves the indented natural either at the girdle or pavilion of the stone, in order to keep it less noticeable. In such positions, the natural is not visible even with a loupe. Indents can be removed if the cutter polishes out rougher. However, this would result in a drop of the diamond's weight by up to 25%.

Carbons

Diamonds are made from carbon, usually graphite. Nevertheless, while a diamond is being formed, it may not totally crystallize leading to the presence of small dots of black carbon. These black spots have been classified to be those of graphite, pyrrhotite and pentlandite. These surface flaws resemble a small black dot and may affect the clarity of the stone depending on the size of imperfection. The occurrence of this kind of flaw is rare in diamonds as compared to pinpoint inclusions. Carbons are usually seen in white or blue-white stones. However carbons are not commonly found in diamonds of poorer colors.

Chips

The breaking off of a small piece of diamond towards the surface is usually referred to as chipping. The term may be confused with 'diamond chips' which refer to very small pieces of diamonds. These are usually caused due to minor impact from the environment. Downward impact caused when a

stone is being set or is being worn, can cause chips on the culet of the diamond. As these are commonly caused when a diamond is worn, it is suggested that while diamonds are being set, a little space be left between the base of the diamond and the head of the prongs of the ring. This space acts as a cushion protecting the diamond from possible chipping when it falls. However, Chips are easy to remove by treating the diamond.

Internal flaws Every natural diamond crystal contains impurities and typical intrinsic or self-defects: vacancies, dislocations, and interstitial atoms. The most common impurity in diamond is nitrogen, which can comprise up to 1% of a diamond by mass. Nitrogen as a diamond impurity was first identified in 1959 by Kaiser and Bond of Bell Telephone.[1]

Crystal/mineral inclusions

An inclusion is visible in near the center of an uncut diamond

Some diamonds show the presence of small crystals, minerals or other diamonds. These are classified in various categories depending upon the size and structure of the inclusion. While many such inclusions are small and not visible to the naked eye, some diamonds may have large inclusions, which can be seen with the naked eye and can affect a diamond's clarity and also its life. Some crystals resemble a diamond inside a diamond and may also add to the look of the stone. These take on shapes of bubbles, needles or grains and are classified as below.

Pinpoint inclusions

As the name implies, these inclusions are minute crystals usually white in color present inside the diamond. These resemble a small point of light and are, by far, the most common of all flaws found in diamonds. Most pinpoint inclusions do not affect the clarity of a diamond and are not visible to the naked eye and are usually not indicated on the plotting diagrams of diamond reports. Comments such as pinpoints not shown may be listed in the comments section.

Needles

Diamond crystals in a diamond can also be present in the form of long and thin needles. These may not be visible to the naked eye, unless the needle inclusion is of a noticeable color or has a noticeable presence. Some needle inclusions are also known to give diamonds a special look.

Cloud

The presence of three or more pinpoint inclusions close together can create an area of haze or a cloud in the diamond. While the occurrence of a small cloud is not visible to the naked eye, presence of many pinpoints covering a large area can affect the clarity of the diamond. These are usually indicated on grading reports in the form of tiny red dots close together or as circles and other formations.

Knots

When diamond crystals extend to the surface of the diamond, they are referred to as knots. These can be viewed under proper lighting conditions with a diamond loupe. Certain knot formations may also cause raised areas on particular facets of the diamond. The presence of knots may affect both the clarity and durability of the diamond and are best avoided.

Grain lines

Crystal inclusions in diamonds can also occur in the form of lines, known as grain lines. These are usually formed due to improper crystallization of the diamond, when it was being formed. Grain lines can also be caused due to improper polishing of the diamond. Even skilled diamond cutters may come across diamonds with variations in hardness when a facet is polished. This can cause microscopic lines across the facet. These grains are usually difficult to remove without excessive weight loss. Grain lines are commonly seen in pink fancy diamonds. A saturation of grain lines on pink stones can also make them look red.

Feathers

These are cracks in the stone that resemble the design of feathers. Presence of this in a diamond usually does not affect the life of the stone unless and until the feather runs through a major length of the stone or shows major stress points where it can break. If the cracks reach the surface or have deep fissures, the durability of the stone may be reduced with the possibility of the stone breaking with age.

Intergrowths

Twinning wisps or intergrowths may also be seen in diamonds. These formations are usually inclusions in diamonds that have twisted together during the time of diamond formation. Thus various inclusions like pinpoints, needles or feathers may form together creating a white strip inside the diamond. Surface graining may also be seen in some cases. Such intergrowths are more commonly seen in fancy shaped diamonds and are extremely rare in ideal cut diamonds.

Cleavage

These are cracks in a diamond that occur in a straight line and are parallel to one of the diamond's crystallographic planes. Cleavages are usually caused by deep internal strain in a diamond and could also have been caused by a strong blow on the diamond. It usually shows no feathers and has a great chance of causing the stone to split, especially if placed in the high pressure

grip of prongs in rings. Stones with cleavage must be chosen carefully and avoided as far as possible.

Bearding

Also known as girdle fencing or 'dig marks', this is caused around the diamond's girdle as the diamond is cut or bruited. These fine lines usually resemble a hair strand and do not present a problem. However extensive bearding can lessen the brightness of the diamond. It is suitable that such diamonds be cut or polished again to improve luster.

Diamond flaws are not always a negative phrase. In fact it is these flaws that often lend a diamond its distinctive beauty. It is often these flaws that make a stone look unique.

Five Factors of Flaw Valuation / The Important factors of clarity grading

Size of the flaw The size of the flaw is crucial in evaluating the clarity grade as well as value of the diamond. Larger the flaw present in the diamond more it steeps down in its clarity grade. The bigger flaw not has more chances of being visible, hampering the brilliance but is also equally difficult to hide. Number The clarity grading is also assessed by the number of flaws present in the diamond. The more the less the clarity, brilliance and value of the diamond, more so if they are visible. Since the diamonds are valued for their crystal clear look, the higher number of flaws kills its aura. Position The position where the flaw is situated largely affects the clarity grading, value and life of the diamond. The internal flaws present under the table and close to the pavilion facet are named reflect. These reflectors have a direct outcome on the diamond clarity grade. Inclusions are less conspicuous when they are positioned under the crown facets or close to the girdle of the stone. Inclusions located at certain crucial areas might even cause the diamond to break. Nature The nature of the flaw-internal or external, determines its value and the possibility of correction. Certain internal flaws may not be visible to the naked eyes or be hidden by cutting and metal setting. Even the minor surface flaws be corrected by polishing, without causing much impact on value. The nature will also throw light on the inclusions that might threaten the anatomy of the stone. Color or relief The amount to which color or relief of the inclusions are noticeable, determines the clarity grade of the diamond. Relief refers to the contrasting characteristics that contrasts the surroundings of the diamond. Colored inclusions show contrast and more easily visible. Though an exception, strikingly black pinpoints are less evident compared to white pinpoints.

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Diamonds Clarity Grade Standards

Diamond clarity largely influences the diamonds value and brilliance. The flaw may or may not be visible to the naked eyes. Gemologist use 10X loupe magnification to evaluate the diamond grade. The grading is based on the industry standards, benchmarked by the Gemological Institute of America (GIA). The most reputed laboratory they have scaled eleven different grades in six categories, to evaluate the diamond’s clarity. In 1952 a team of gemologist lead by Richard T Liddicoat started working on the diamond grading system, with only nine clarity grades. They defined- flawless, VVS1, VVS2, VS1, VS2, SI1, SI2, I1 and I2, where I in I1 and I2 grades stood for word Imperfect. Then latter in 1970s two changes were made to the system. Internally Flawless (IF)

grade were added- as GIA realized that diamonds were precariously cut, adding to the blemishes. The second change was the origination of the I3 grade. Rising number of low clarity diamonds gave birth to this alteration. The last modification took place in 1990s when term ‘Imperfect’ was updated to, ‘Included’. The GIA grading scale is as follows:-

Category Flawless Internally Flawless

Very-Very Slightly Included

Very Slightly Included

Slightly Included Included

Grade FL IF VVS1 VVS2 VS1 VS2 SI1 SI2 I1 I2 I3

These GIA diamond grading scale can be further understood as follows:- FL-Flawless: The diamonds in this grade do not show any internal or external flaws, under the 10x loupe magnification. IF - Internally Flawless: The diamonds that fall in this category have no internal flaws and only slight external blemishes. VVS – Very-Very Slightly Included The diamonds that fall under this grade have minute inclusions that are very difficult even for a skilled grader to see under, 10X magnification. Pinpoints and needles grade at VVS. The VVS category is further divided into two grades. VVS1- These are excellent quality diamonds. The miniscule inclusions may not even be visible under 10X magnification. VVS1 is better than VVS2. VVS2: Beautiful brilliance and high quality, these stones have very, very small inclusions. VSI: Very Slightly Included category These diamonds come with minor inclusions that are hardly visible under 10X magnifications. Generally, the inclusions present in VS diamonds are invisible without the magnification. The VS category is divided into two grades. VS1- The inclusions are not at all visible to the naked eyes though can be traced under 10X magnification. They make a good purchase with better quality with value.VS1 is better than VS2. VS2- These stones have very small inclusions. Rarely there is possibility that diamonds that fall under this grade have inclusions visible to the eye. SI- Slightly Included category The diamonds that fall under this category have evident inclusions that are visible to the trained grader under 10X loupe magnification. Diamonds in this grade are good in quality and less expensive. Primarily, SI category is divided into two grades. SI1- The diamonds in this grade have inclusions that may or may not be noticeable to the naked eye. SI1 denotes a higher clarity grade than SI2. SI2: - These stones have inclusions that may be visible with the naked eye.

I -Included category Diamonds have obvious inclusions that are clearly visible to the trained gemologist even without magnification. These inclusions may be easily detectable without magnification affecting the brilliance and the durability of the stone. ‘I’ category is divided into three grades. I1 refers to a higher clarity grade than I2, which in turn is higher than I3. I2-I3 The diamond belonging to I2-3 are the lowest grades having observable inclusions. I1- Inclusions present in the diamonds falling in this grade are visible to the unaided eye. I2- Diamonds in this grade have inclusions that are noticeably visible. I3- The inclusions in the diamonds of this grade are quite conspicuous, affect the brilliance of the diamond and threaten the structure of the diamond. *MySolitaire does not carry any diamonds below I1 clarity. * All MySolitaire loose diamonds are graded by GIA.

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Grading standards of other Laboratories organizations

Different organizations grade the diamond clarity by subjecting the diamond to 10x magnification. Their skilled professional study the diamond characteristic, under special environment and lighting. However, the grading scales are usually the same, with minor additions or variations. American Gem Society- Diamond Grading Scale

AGS diamond clarity grading scale

GIA FL IF VVS1 VVS2 VS1 VS2 SI1 SI2 I1 I2 I3

AGS 0 1 2 3 4 5 6 7 8 9 10

The American Gem Society (AGS) ranks diamond clarity on a number scale between 0 and 10. These numbers grades are virtually correlative with the GIA system, but with some minor changes. One of the most respected labs, they have clubbed the Flawless (FL) and Internally Flawless (IF) represented in the (0) grade with notation specifying if the stone is clear of external blemishes, the VVS through SI grades are numerated 1 through 6, and then there are four grades 7 through 10 for the category. International Diamond Council (IDC) - Diamond Grading Scale

IDC diamond clarity grading scale

GIA FL IF VVS1 VVS2 VS1 VS2 SI1 SI2 I1 I2 I3

IDC Loupe clean VVS1 VVS2 VS1 VS2 SI1 SI2 PI PII PIII

The standards set in by the IDC define Flawless (FL) as Loupe Clean diamonds. The surface blemishes are specified on their grading report. The European Gemological Laboratory (EGL) on the other hand have initiated a new SI3 as a clarity grade. The aim was to include the borderline SI2/I1 stones, presently it is used to refer to eye-clean it I1 stones that have inclusions not visible to unaided eye.

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Clarity Grading Procedure

Gemologists and grading labs use different processes and standards to evaluate the diamond grade. GIA uses 10X magnification with darkfield illumination to determine the diamond clarity and its grading. The grading is executed using 10x handheld loupe under ultraviolet light, filtered with overhead light. The light-source is used to illuminate the base of the stone and crown of the stone concealed from the light. After carefully cleaning the diamond, the latter is lifted with tweezers. The inclusions in the diamond are studied from the pavilion side as well as the crown side of the stone, scanning through each facet. If the grader is using a stereo microscope, he can zoom in to a higher magnification to make closer observations of an inclusion. He needs to again zoom out, back to 10x magnification to make a judgment of its impact on the clarity.

10X diamond grading loupe is the standard equipment used for the diamond grading. It is the basic degree of magnification to should rely on when evaluating a diamond for purchase. The higher magnifying equipments like 20X, 30x to 70X, may show more inclusions which may not be as clear under the 10X industry standards. Since no diamond is perfect, it is possible that even a

Flawless (FL) diamond may show some inclusions under higher magnification. Back to home

Clarity and Rarity v/s Value

Diamonds that have highest clarity are few and far between. Merely 20 percent of the total diamonds mined posses the clarity rating that would ensure their use as a gemstone, remaining 80 percent are submitted to industrial use. Of that 20 percent a substantial lot contains inclusions and/or blemishes conspicuous even to the unaided eyes. Only one-fourth of the gem-quality diamonds can be graded Flawless under 10X loupe. Consequently, those that are visibly flawless are greatly desired in the market. The subsequent rarity against the high demand and diamond aesthetic value makes it an extremely valuable treasure. In fact, the diamonds that fall within the VS and SI grades with FL, IF and VVS; the FL and IF diamonds are also known as ‘museum quality’ or ‘investment grade’.

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Clarity Enhancement

The market is filled with diamonds that do not approve the GIA standards and use unlawful treatments to enhance the diamond clarity. Ensure that you too do not end up being cheated paying more for a diamond that is even below the acceptable standards. It is therefore advised to always buy a diamond from a reputed jewelry store like MySolitaire.

The popular practices for clarity enhancement of the diamonds include- Laser Treatments The laser can be used to remove the inclusions present in the diamond. An experienced gemologist can however detect the laser marking. This treatment often affects the florescence of the diamond. The changes left behind by the laser are permanent. The diamond’s clarity is enhanced by laser “drilling” that employs laser to burn a hole to a colored inclusion succeeded by acid washing to remove the coloring agent. MySolitaire does not offer such diamonds. Diamond Fracture Filling Minute cracks in the diamond are filled with colorless substance. This is an unlawful practice with temporary results. Clarity can be improved by filling the diamond. These diamonds aka fracture filled diamonds shows some color tinge commonly orange, pink. In fact, certain genuine renowned companies often facilitate repeat treatments if the heat ruptures the filling. These clarity enhancement is not approved by the GIA, nor do they grade fracture-filled diamond. If the GIA diamonds that have notification of "clarity enhanced" or "fracture-filled," it is a fake report.

Synthetic diamond (also known as laboratory-created diamond, laboratory-grown diamond, cultured diamond or cultivated diamond) is diamond produced in an artificial process, as opposed

to natural diamonds, which are created by geological processes. Synthetic diamond is also widely known as HPHT diamond or CVD diamond after the two common production methods (referring to the high-pressure high-temperature and chemical vapor deposition crystal formation methods, respectively).

Although often referred to as synthetic, this term has been considered somewhat problematic. In the U.S., the Federal Trade Commission has indicated that the alternative terms laboratory-grown, laboratory-created, and [manufacturer-name]-created "would more clearly communicate the nature of the stone", as consumers associate the term synthetic with imitation products – whereas man-made diamonds are actual diamond material.[1]

Numerous claims of diamond synthesis were documented between 1879 and 1928; most of those attempts were carefully analyzed but none were confirmed. In the 1940s, systematic research began in the United States, Sweden and the Soviet Union to grow diamonds using CVD and HPHT processes. The first reproducible synthesis was reported around 1953. Those two processes still dominate the production of synthetic diamond. A third method, known as detonation synthesis, entered the diamond market in the late 1990s. In this process, nanometer-sized diamond grains are created in a detonation of carbon-containing explosives. A fourth method, treating graphite with high-power ultrasound, has been demonstrated in the laboratory, but currently has no commercial application.

The properties of synthetic diamond depend on the details of the manufacturing processes; however, some synthetic diamonds (whether formed by HPHT or CVD) have properties such as hardness, thermal conductivity and electron mobility that are superior to those of most naturally-formed diamonds.

Synthetic diamond is widely used in abrasives, in cutting and polishing tools and in heat sinks. Electronic applications of synthetic diamond are being developed, including high-power switches at power stations, high-frequency field-effect transistors and light-emitting diodes. Synthetic diamond detectors of ultraviolet (UV) light or high-energy particles are used at high-energy research facilities and are available commercially. Because of its unique combination of thermal and chemical stability, low thermal expansion and high optical transparency in a wide spectral range, synthetic diamond is becoming the most popular material for optical windows in high-power CO2 lasers and gyrotrons.

Both CVD and HPHT diamonds can be cut into gems and various colors can be produced: clear white, yellow, brown, blue, green and orange. The appearance of synthetic gems on the market created major concerns in the diamond trading business, as a result of which special spectroscopic devices and techniques have been developed to distinguish synthetic and natural diamonds.

Contents • 1 History

o 1.1 GE diamond project o 1.2 Later developments

• 2 Manufacturing technologies o 2.1 High pressure, high temperature o 2.2 Chemical vapor deposition

o 2.3 Detonation of explosives o 2.4 Ultrasound cavitation

• 3 Properties o 3.1 Crystallinity o 3.2 Hardness o 3.3 Impurities and inclusions o 3.4 Thermal conductivity

• 4 Applications o 4.1 Machining and cutting tools o 4.2 Thermal conductor o 4.3 Optical material o 4.4 Electronics o 4.5 Gemstones

• 5 See also • 6 References • 7 Bibliography • 8 External links

History

Moissan trying to create synthetic diamonds using an electric arc furnace

After the 1797 discovery that diamond was pure carbon, many attempts were made to convert various cheap forms of carbon into diamond. The earliest successes were reported by James Ballantyne Hannay in 1879[2] and by Ferdinand Frédéric Henri Moissan in 1893. Their method involved heating charcoal at up to 3500 °C with iron inside a carbon crucible in a furnace. Whereas Hannay used a flame-heated tube, Moissan applied his newly developed electric arc furnace, in which an electric arc was struck between carbon rods inside blocks of lime.[3] The molten iron was then rapidly cooled by immersion in water. The contraction generated by the cooling supposedly produced the high pressure required to transform graphite into diamond. Moissan published his work in a series of articles in the 1890s.[4]

Many other scientists tried to replicate his experiments. Sir William Crookes claimed success in 1909.[5] Otto Ruff claimed in 1917 to have produced diamonds up to 7 mm in diameter,[6] but later retracted his statement.[7] In 1926, Dr. J.Willard Hershey of McPherson College replicated Moissan's and Ruff's experiments,[8][9] producing a synthetic diamond; that specimen is on display at

the McPherson Museum in Kansas.[10] Despite the claims of Moissan, Ruff, and Hershey, other experimenters were unable to reproduce their synthesis.[11][12]

The most definitive replication attempts were performed by Sir Charles Algernon Parsons. A prominent scientist and engineer known for his invention of the steam turbine, he spent about 40 years (1882–1922) and a considerable part of his fortune trying to reproduce the experiments of Moissan and Hannay, but also adapted processes of his own.[13] Parsons was known for his painstakingly accurate approach and methodical record keeping; all his resulting samples were preserved for further analysis by an independent party.[14] He wrote a number of articles—some of the earliest on HPHT diamond—in which he claimed to have produced small diamonds.[15] However in 1928 he authorized Dr. C.H. Desch to publish an article[16] in which he stated his belief that no synthetic diamonds (including those of Moissan and others) had been produced up to that date. He suggested that most diamonds that had been produced up to that point were likely synthetic spinel.[11]

GE diamond project

A belt press produced in the 1980s by KOBELCO

In 1941, an agreement was made between the General Electric (GE), Norton and Carborundum companies to further develop diamond synthesis. They were able to heat carbon to about 3,000 °C (5,430 °F) under a pressure of 3.5 gigapascals (510,000 psi) for a few seconds. Soon thereafter the Second World War interrupted the project. It was resumed in 1951 at the Schenectady Laboratories of GE, and a high-pressure diamond group was formed with F.P. Bundy and H.M. Strong. Tracy Hall and others joined this project shortly thereafter.[17]

The Schenectady group improved on the anvils designed by Percy Bridgman, who received a Nobel Prize for his work in 1946. Bundy and Strong made the first improvements, then more were made by Hall. The GE team used tungsten carbide anvils within a hydraulic press to squeeze the carbonaceous sample held in a catlinite container, the finished grit being squeezed out of the container into a gasket. The team recorded diamond synthesis on one occasion, but the experiment could not be reproduced because of uncertain synthesis conditions,[18] and the diamond was later shown to have been a natural diamond used as a seed.[19]

Hall achieved the first commercially successful synthesis of diamond on December 16, 1954, and this was announced on February 15, 1955. His breakthrough was using a "belt" press, which was capable of producing pressures above 10 GPa (1,500,000 psi) and temperatures above 2,000 °C (3,630 °F).[20] The press used a pyrophyllite container in which graphite was dissolved within molten nickel, cobalt or iron. Those metals acted as a "solvent-catalyst", which both dissolved carbon and accelerated its conversion into diamond. The largest diamond he produced was 0.15 mm (0.0059 in) across; it was too small and visually imperfect for jewelry, but usable in industrial abrasives. Hall's co-workers were able to replicate his work, and the discovery was published in the major journal Nature.[21][22] He was the first person to grow a synthetic diamond with a reproducible, verifiable and well-documented process. He left GE in 1955, and three years later developed a new apparatus for the synthesis of diamond—a tetrahedral press with four anvils—to avoid violating a U.S. Department of Commerce secrecy order on the GE patent applications.[19][23] Hall received the American Chemical Society Award for Creative Invention for his work in diamond synthesis.[24]

Later developments

An independent diamond synthesis was achieved on February 16, 1953 in Stockholm by the ASEA (Allmänna Svenska Elektriska Aktiebolaget), one of Sweden's major electrical manufacturing companies. Starting in 1949, ASEA employed a team of five scientists and engineers as part of a top-secret diamond-making project code-named QUINTUS. The team used a bulky split-sphere apparatus designed by Baltzar von Platen and Anders Kämpe.[17][25] Pressure was maintained within the device at an estimated 8.4 GPa for an hour. A few small diamonds were produced, but not of gem quality or size. The work was not reported until the 1980s.[26] During the 1980s, a new competitor emerged in Korea, a company named Iljin Diamond; it was followed by hundreds of Chinese enterprises. Iljin Diamond allegedly accomplished diamond synthesis in 1988 by misappropriating trade secrets from GE via a Korean former GE employee.[27][28]

A scalpel with single-crystal synthetic diamond blade

Synthetic gem-quality diamond crystals were first produced in 1970 by GE, then reported in 1971. The first successes used a pyrophyllite tube seeded at each end with thin pieces of diamond. The graphite feed material was placed in the center and the metal solvent (nickel) between the graphite and the seeds. The container was heated and the pressure was raised to about 5.5 GPa. The crystals grow as they flow from the center to the ends of the tube, and extending the length of the process produces larger crystals. Initially a week-long growth process produced gem-quality stones of around 5 mm (1 carat or 0.2 g), and the process conditions had to be as stable as possible. The

graphite feed was soon replaced by diamond grit because that allowed much better control of the shape of the final crystal.[22]

The first gem-quality stones were always yellow to brown in color because of contamination with nitrogen. Inclusions were common, especially "plate-like" ones from the nickel. Removing all nitrogen from the process by adding aluminium or titanium produced colorless "white" stones, and removing the nitrogen and adding boron produced blue ones.[29] Removing nitrogen also slowed the growth process and reduced the crystalline quality, so the process was normally run with nitrogen present.

Although the GE stones and natural diamonds were chemically identical, their physical properties were not the same. The colorless stones produced strong fluorescence and phosphorescence under short-wavelength ultraviolet light, but were inert under long-wave UV. Among natural diamonds, only the rarer blue gems exhibit these properties. Unlike natural diamonds, all the GE stones showed strong yellow fluorescence under X-rays.[30] The De Beers Diamond Research Laboratory has grown stones of up to 25 carats (5.0 g) for research purposes. Stable HPHT conditions were kept for six weeks to grow high-quality diamonds of this size. For economic reasons, the growth of most synthetic diamonds is terminated when they reach a mass of 1 carat (200 mg) to 1.5 carats (300 mg).[31]

In the 1950s, research started in the Soviet Union and the US on the growth of diamond by pyrolysis of hydrocarbon gases at the relatively low temperature of 800 °C. This low-pressure process is known as chemical vapor deposition (CVD). William G. Eversole reportedly achieved vapor deposition of diamond over diamond substrate in 1953, but it was not reported until 1962.[32] Diamond film deposition was independently reproduced by Angus and coworkers in 1968[33] and by Deryagin and Fedoseev in 1970.[34] Whereas Eversole and Angus used large, expensive, single-crystal diamonds as substrates, Deryagin and Fedoseev succeeded in making diamond films on non-diamond materials (silicon and metals), which led to massive research on inexpensive diamond coatings in the 1980s.[35]

Manufacturing technologies There are several methods used to produce synthetic diamond. The original method uses high pressure and high temperature (HPHT) and is still widely used because of its relatively low cost. The process involves large presses that can weigh hundreds of tons to produce a pressure of 5 GPa at 1500 °C. The second method, using chemical vapor deposition (CVD), creates a carbon plasma over a substrate onto which the carbon atoms deposit to form diamond. Other methods include explosive formation (forming detonation nanodiamonds) and sonication of graphite solutions.[36][37][38]

High pressure, high temperature

Schematic of a belt press

In the HPHT method, there are three main press designs used to supply the pressure and temperature necessary to produce synthetic diamond: the belt press, the cubic press and the split-sphere (BARS) press.

The original GE invention by Tracy Hall uses the belt press wherein the upper and lower anvils supply the pressure load to a cylindrical inner cell. This internal pressure is confined radially by a belt of pre-stressed steel bands. The anvils also serve as electrodes providing electrical current to the compressed cell. A variation of the belt press uses hydraulic pressure, rather than steel belts, to confine the internal pressure.[39] Belt presses are still used today, but they are built on a much larger scale than those of the original design.[40]

The second type of press design is the cubic press. A cubic press has six anvils which provide pressure simultaneously onto all faces of a cube-shaped volume.[41] The first multi-anvil press design was a tetrahedral press, using four anvils to converge upon a tetrahedron-shaped volume.[42] The cubic press was created shortly thereafter to increase the volume to which pressure could be applied. A cubic press is typically smaller than a belt press and can more rapidly achieve the pressure and temperature necessary to create synthetic diamond. However, cubic presses cannot be easily scaled up to larger volumes: the pressurized volume can be increased by using larger anvils, but this also increases the amount of force needed on the anvils to achieve the same pressure. An alternative is to decrease the surface area to volume ratio of the pressurized volume, by using more anvils to converge upon a higher-order platonic solid, such as a dodecahedron. However, such a press would be complex and difficult to manufacture.[41]

Schematic of a BARS system; the size of the outer barrel is reduced for presentation purposes

The BARS apparatus is the most compact, efficient, and economical of all the diamond-producing presses. In the center of a BARS device, there is a ceramic cylindrical "synthesis capsule" of about 2 cm3 in size. The cell is placed into a cube of pressure-transmitting material, such as pyrophyllite ceramics, which is pressed by inner anvils made from cemented carbide (e.g., tungsten carbide or VK10 hard alloy).[43] The outer octahedral cavity is pressed by 8 steel outer anvils. After mounting, the whole assembly is locked in a disc-type barrel with a diameter about 1 meter. The barrel is filled with oil, which pressurizes upon heating, and the oil pressure is transferred to the central cell. The synthesis capsule is heated up by a coaxial graphite heater and the temperature is measured with a thermocouple.[44]

Chemical vapor deposition

Chemical vapor deposition is a method by which diamond can be grown from a hydrocarbon gas mixture. Since the early 1980s, this method has been the subject of intensive worldwide research. Whereas the mass-production of high-quality diamond crystals make the HPHT process the more suitable choice for industrial applications, the flexibility and simplicity of CVD setups explain the popularity of CVD growth in laboratory research. The advantages of CVD diamond growth include the ability to grow diamond over large areas and on various substrates, and the fine control over the chemical impurities and thus properties of the diamond produced. Unlike HPHT, CVD process does not require high pressures, as the growth typically occurs at pressures under 27 kPa.[36][45]

The CVD growth involves substrate preparation, feeding varying amounts of gases into a chamber and energizing them. The substrate preparation includes choosing an appropriate material and its crystallographic orientation; cleaning it, often with a diamond powder to abrade a non-diamond substrate; and optimizing the substrate temperature (about 800 °C) during the growth through a series of test runs. The gases always include a carbon source, typically methane, and hydrogen with a typical ratio of 1:99. Hydrogen is essential because it selectively etches off non-diamond carbon. The gases are ionized into chemically active radicals in the growth chamber using microwave power, a hot filament, an arc discharge, a welding torch, a laser, an electron beam, or other means.

During the growth, the chamber materials are etched off by the plasma and can incorporate into the growing diamond. In particular, CVD diamond is often contaminated by silicon originating from the silica windows of the growth chamber or from the silicon substrate.[46] Therefore, silica windows are either avoided or moved away from the substrate. Boron-containing species in the chamber, even at very low trace levels, also make it unsuitable for the growth of pure diamond.[36][45][47]

Detonation of explosives

Electron micrograph (TEM) of detonation nanodiamond

Main article: Detonation nanodiamond

Diamond nanocrystals (5 nm in diameter) can be formed by detonating certain carbon-containing explosives in a metal chamber. These nanocrystals are called "detonation nanodiamond". During the explosion, the pressure and temperature in the chamber become high enough to convert the carbon of the explosives into diamond. Being immersed in water, the chamber cools rapidly after the explosion, suppressing conversion of newly produced diamond into more stable graphite.[48] In a variation of this technique, a metal tube filled with graphite powder is placed in the detonation chamber. The explosion heats and compresses the graphite to an extent sufficient for its conversion into diamond.[49] The product is always rich in graphite and other non-diamond carbon forms and requires prolonged boiling in hot nitric acid (about 1 day at 250 °C) to dissolve them.[37] The recovered nanodiamond powder is used primarily in polishing applications. It is mainly produced in China, Russia and Belarus and started reaching the market in bulk quantities by the early 2000s.[50]

Ultrasound cavitation

Micron-sized diamond crystals can be synthesized from a suspension of graphite in organic liquid at atmospheric pressure and room temperature using ultrasonic cavitation. The diamond yield is about 10% of the initial graphite weight. The estimated cost of diamond produced by this method is comparable to that of the HPHT method; the crystalline perfection of the product is significantly worse for the ultrasonic synthesis. This technique requires relatively simple equipment and procedures, but it has only been reported by two research groups, and has no industrial use as of 2012. Numerous process parameters, such as preparation of the initial graphite powder, the choice of ultrasonic power, synthesis time and the solvent, are not yet optimized, leaving a window for potential improvement of the efficiency and reduction of the cost of the ultrasonic synthesis.[38][51]

Properties Traditionally, the absence of crystal flaws is considered to be the most important quality of a diamond. Purity and high crystalline perfection make diamonds transparent and clear, whereas its hardness, optical dispersion (luster) and chemical stability (combined with marketing), make it a popular gemstone. High thermal conductivity is also important for technical applications. Whereas

high optical dispersion is an intrinsic property of all diamonds, their other properties vary depending on how the diamond was created.[52]

Crystallinity

Diamond can be one single, continuous crystal or it can be made up of many smaller crystals (polycrystal). Large, clear and transparent single-crystal diamonds are typically used in gemstones. Polycrystalline diamond consists of numerous small grains, which are easily seen by the naked eye through strong light absorption and scattering; it is unsuitable for gems and is used for industrial applications such as mining and cutting tools. Polycrystalline diamond is often described by the average size (or grain size) of the crystals that make it up. Grain sizes range from nanometers to hundreds of micrometers, usually referred to as "nanocrystalline" and "microcrystalline" diamond, respectively.[53]

Hardness

Synthetic diamond is the hardest material known,[54] where hardness is defined as resistance to scratching and is graded between 1 (softest) and 10 (hardest) using the Mohs scale of mineral hardness. Diamond has a hardness of 10 (hardest) on this scale.[55] The hardness of synthetic diamond depends on its purity, crystalline perfection and orientation: hardness is higher for flawless, pure crystals oriented to the [111] direction (along the longest diagonal of the cubic diamond lattice).[56] Nanocrystalline diamond produced through CVD diamond growth can have a hardness ranging from 30% to 75% of that of single crystal diamond, and the hardness can be controlled for specific applications. Some synthetic single-crystal diamonds and HPHT nanocrystalline diamonds (see hyperdiamond) are harder than any known natural diamond.[54][57][58]

Impurities and inclusions

Main article: Crystallographic defects in diamond

Every diamond contains atoms other than carbon in concentrations detectable by analytical techniques. Those atoms can aggregate into macroscopic phases called inclusions. Impurities are generally avoided, but can be introduced intentionally as a way to control certain properties of the diamond. For instance, pure diamond is an electrical insulator, but diamond with boron added is an electrical conductor (and, in some cases, a superconductor),[59] allowing it to be used in electronic applications. Nitrogen impurities hinder movement of lattice dislocations (defects within the crystal structure) and put the lattice under compressive stress, thereby increasing hardness and toughness.[60]

Thermal conductivity

Unlike most electrical insulators, pure diamond is a good conductor of heat because of the strong covalent bonding within the crystal. The thermal conductivity of pure diamond is the highest of any known solid. Single crystals of synthetic diamond enriched in 12C (99.9%), isotopically pure diamond, have the highest thermal conductivity of any material, 30 W/cm·K at room temperature, 7.5 times higher than copper. Natural diamond's conductivity is reduced by 1.1% by the 13C naturally present, which acts as an inhomogeneity in the lattice.[61]

Diamond's thermal conductivity is made use of by jewelers and gemologists who may employ an electronic thermal probe to separate diamonds from their imitations. These probes consist of a pair of battery-powered thermistors mounted in a fine copper tip. One thermistor functions as a heating device while the other measures the temperature of the copper tip: if the stone being tested is a diamond, it will conduct the tip's thermal energy rapidly enough to produce a measurable temperature drop. This test takes about 2–3 seconds.[62]

Applications

Machining and cutting tools

Diamonds in an angle grinder blade

Most industrial applications of synthetic diamond have long been associated with their hardness; this property makes diamond the ideal material for machine tools and cutting tools. As the hardest known naturally occurring material, diamond can be used to polish, cut, or wear away any material, including other diamonds. Common industrial applications of this ability include diamond-tipped drill bits and saws, and the use of diamond powder as an abrasive.[63] These are by far the largest industrial applications of synthetic diamond. While natural diamond is also used for these purposes, synthetic HPHT diamond is more popular, mostly because of better reproducibility of its mechanical properties. Diamond is not suitable for machining ferrous alloys at high speeds, as carbon is soluble in iron at the high temperatures created by high-speed machining, leading to greatly increased wear on diamond tools compared to alternatives.[64]

The usual form of diamond in cutting tools is micrometer-sized grains dispersed in a metal matrix (usually cobalt) sintered onto the tool. This is typically referred to in industry as polycrystalline diamond (PCD). PCD-tipped tools can be found in mining and cutting applications. For the past fifteen years, work has been done to coat metallic tools with CVD diamond, and though the work still shows promise it has not significantly replaced traditional PCD tools.[65]

Thermal conductor

Most materials with high thermal conductivity are also electrically conductive, such as metals. In contrast, pure synthetic diamond has high thermal conductivity, but negligible electrical conductivity. This combination is invaluable for electronics where diamond is used as a heat sink for high-power laser diodes, laser arrays and high-power transistors. Efficient heat dissipation prolongs

the lifetime of those devices, and their high cost justifies the use of efficient, though relatively expensive, diamond heat sinks.[66] In semiconductor technology, synthetic diamond heat spreaders prevent silicon and other semiconducting materials from overheating.[67]

Optical material

Diamond is hard, chemically inert, and has high thermal conductivity and a low coefficient of thermal expansion. These properties make diamond superior to any other existing window material used for transmitting infrared and microwave radiation. Therefore, synthetic diamond is starting to replace zinc selenide as the output window of high-power CO2 lasers[68] and gyrotrons. Those synthetic diamond windows are shaped as disks of large diameters (about 10 cm for gyrotrons) and small thicknesses (to reduce absorption) and can only be produced with the CVD technique.[69][70]

Recent advances in the HPHT and CVD synthesis techniques improved the purity and crystallographic structure perfection of single-crystalline diamond enough to replace silicon as a diffraction grating and window material in high-power radiation sources, such as synchrotrons.[71][72] Both the CVD and HPHT processes are also used to create designer optically transparent diamond anvils as a tool for measuring electric and magnetic properties of materials at ultra high pressures using a diamond anvil cell.[73]

Electronics

Synthetic diamond has potential uses as a semiconductor,[74] because it can be doped with impurities like boron and phosphorus. Since these elements contain one more or one less valence electron than carbon, they turn synthetic diamond into p-type or n-type semiconductor. Making a p–n junction by sequential doping of synthetic diamond with boron and phosphorus produces light-emitting diodes (LEDs) producing UV light of 235 nm.[75] Another useful property of synthetic diamond for electronics is high carrier mobility, which reaches 4500 cm2/(V·s) for electrons in single-crystal CVD diamond.[76] High mobility is favorable for high-frequency field-effect transistors. The wide band gap of diamond (5.5 eV) gives it excellent dielectric properties. Combined with the high mechanical stability of diamond, those properties are being used in prototype high-power switches for power stations.[77]

Synthetic diamond transistors have been produced in the laboratory. They are functional at much higher temperatures than silicon devices, and are resistant to chemical and radiation damage. While no diamond transistors have yet been successfully integrated into commercial electronics, they are promising for use in exceptionally high power situations and hostile non-oxidizing environments.[78][79]

Synthetic diamond is already used as radiation detection device. It is radiation hard and has a wide bandgap of 5.5 eV (at room temperature). Diamond is also distinguished from most other semiconductors by the lack of a stable native oxide. This makes it difficult to fabricate surface MOS devices but does create the potential for UV radiation to get to the active semiconductor without absorption in a surface layer. Because of these properties, it is employed in applications such as the BaBar detector at the Stanford Linear Accelerator[80] and BOLD (Blind to the Optical Light Detectors for VUV solar observations).[81][82] A diamond VUV detector recently was used in the European LYRA program.

Conductive CVD diamond is a useful electrode under many circumstances.[83] Photochemical methods have been developed for covalently linking DNA to the surface of polycrystalline diamond films produced through CVD. Such DNA modified films can be used for detecting various biomolecules, which would interact with DNA thereby changing electrical conductivity of the diamond film.[84] In addition, diamonds can be used to detect redox reactions that cannot ordinarily be studied and in some cases degrade redox-reactive organic contaminants in water supplies. Because diamond is mechanically and chemically stable, it can be used as an electrode under conditions that would destroy traditional materials. As an electrode, synthetic diamond can be used in waste water treatment of organic effluents[85] and the production of strong oxidants.[86]

Gemstones

Colorless gem cut from diamond grown by chemical vapor deposition

Synthetic diamonds for use as gemstones are grown by HPHT[31] or CVD[87] methods. They are available in yellow and blue, and to a lesser extent colorless (or white). The yellow color comes from nitrogen impurities in the manufacturing process while the blue color comes from boron.[29] Other colors such as pink or green are achievable after synthesis using irradiation.[88] Several companies also offer memorial diamonds grown using cremated remains.[89]

Gem-quality diamonds grown in a lab can be chemically, physically and optically identical (and sometimes superior) to naturally occurring ones. The mined diamond industry has undertaken legal, marketing and distribution countermeasures to protect its market from the emerging presence of synthetic diamonds. Man-made diamonds can be distinguished by spectroscopy in the infrared, ultraviolet, or X-ray wavelengths. The DiamondView tester from De Beers uses UV fluorescence to detect trace impurities of nitrogen, nickel or other metals in HPHT or CVD diamonds.[90]

At least one maker of laboratory-grown diamonds has made public statements about being "committed to disclosure" of the nature of its diamonds, and laser-inscribes serial numbers on all of its gemstones.[87] The company web site shows an example of the lettering of one of its laser inscriptions, which includes both the words "Gemesis created" and the serial number prefix "LG" (laboratory grown).[9

Diamond cutting From Wikipedia, the free encyclopedia

Diamond cutting is the practice of changing a diamond from a rough stone into a faceted gem. Cutting diamond requires specialized knowledge, tools, equipment, and techniques because of its extreme difficulty.

The first guild of diamond cutters and polishers (diamantaire) was formed in 1375 in Nuremberg, Germany, and led to the development of various types of 'cut'. This has two meanings in relation to diamonds. The first is the shape: square, oval, and so on. The second relates to the specific quality of cut within the shape, and the quality and price will vary greatly based on the cut quality. Since diamonds are very hard to cut, special diamond-bladed edges are used to cut them. The first major development in diamond cutting came with the "Point Cut" during the later half of the 14th century: the Point Cut follows the natural shape of an octahedral raw diamond crystal, eliminating some waste in the cutting process.

Diamond cutting, as well as overall processing, is concentrated in a few cities around the world: while 80% of rough diamonds are handled in the Antwerp diamond district in Belgium, more than 50% of processed diamonds also pass through there. Ninety-two percent of diamond pieces are cut in Surat, Gujarat state in India. The other important diamond centers are Tel Aviv and New York City.[1]

Contents • 1 Diamond cutting process

o 1.1 Planning 1.1.1 Maximizing value

1.1.1.1 Weight retention 1.1.1.2 Color retention

1.1.2 Turnaround minimization o 1.2 Cleaving or sawing o 1.3 Bruting o 1.4 Polishing o 1.5 Final inspection

• 2 Cutting process • 3 See also • 4 References • 5 External links

Diamond cutting process The diamond cutting process includes these steps; planning, cleaving or sawing, bruting, polishing, and final inspection.[2]

Planning

Diamond manufacturers analyze diamond rough from an economic perspective, with two objectives steering decisions made about how a faceted diamond will be cut. The first objective is that of maximum return on investment for the piece of diamond rough. The second is how quickly the

finished diamond can be sold. Scanning devices are used to get 3-dimensional computer model of the rough stone. Also, inclusions are photographed and placed on the 3D model, which is then used to find an optimal way to cut the stone.

Maximizing value

Man-powered diamond cutting mill in 18th century

The process of maximizing the value of finished diamonds, from a rough diamond into a polished gemstone, is both an art and a science. The choice of cut is influenced by many factors. Market factors include the exponential increase in value of diamonds as weight increases, referred to as weight retention, and the popularity of certain shapes amongst consumers. Physical factors include the original shape of the rough stone, and location of the inclusions and flaws to be eliminated.

Weight retention

The weight retention analysis studies the diamond rough to find the best combination of finished stones as it relates to per carat value. For instance, a 2.20 carat (440 mg) octahedron may produce (i) either two half carat (100 mg) diamonds whose combined value may be higher than that of (ii) a 0.80 carat (160 mg) diamond + 0.30 carat (60 mg) diamond that could be cut from the same rough diamond.

The round brilliant cut and square brilliant cuts are preferred when the crystal is an octahedron, as often two stones may be cut from one such crystal. Oddly shaped crystals, such as macles are more likely to be cut in a fancy cut—that is, a cut other than the round brilliant—which the particular crystal shape lends itself to.

Even with modern techniques, the cutting and polishing of a diamond crystal always results in a dramatic loss of weight, about 50%.[3] Sometimes the cutters compromise and accept lesser proportions and symmetry in order to avoid inclusions or to preserve the weight. Since the per-carat price of a diamond shifts around key milestones (such as 1.00 carat), many one-carat (200 mg) diamonds are the result of compromising Cut quality for Carat weight. Some jewelry experts[who?] advise consumers to buy a 0.99 carat (198 mg) diamond for its better price or buy a 1.10 carat (220 mg) diamond for its better cut, avoiding a 1.00 carat (200 mg) diamond, which is more likely to be a poorly cut stone.

Color retention

The 253-carat Oppenheimer Diamond—an uncut diamond does not show its prized optical properties.

In colored diamonds, cutting can influence the color grade of the diamond, thereby raising its value. Certain cut shapes are used to intensify the color of the diamond. The radiant cut is an example of this type of cut.

Natural green color diamonds most often have merely a surface coloration caused by natural irradiation, which does not extend through the stone. For this reason green diamonds are cut with significant portions of the original rough diamond's surface (naturals) left on the finished gem. It is these naturals that provide the color to the diamond.

Turnaround minimization

The other consideration of diamond planning is how quickly a diamond will sell. This consideration is often unique to the type of manufacturer. While a certain cutting plan may yield a better value, a different plan may yield diamonds that will sell sooner, and thereby returning the investment sooner.

Cleaving or sawing

Cleaving is the separation of a piece of diamond rough into separate pieces, to be finished as separate gems.

Sawing is the use of a diamond saw or laser to cut the diamond rough into separate pieces.

Bruting

Bruting is the process whereby two diamonds are set onto spinning axles turning in opposite directions, which are then set to grind against each other to shape each diamond into a round shape. This can also be known as girdling.

Polishing

Diamond polisher in Amsterdam

Polishing is the name given to process whereby the facets are cut onto the diamond and final polishing is performed. The process takes the steps blocking, faceting, also called "brillianteering", and polishing.

Final inspection

The final stage involves thoroughly cleaning the diamond in acids, and examining the diamond to see whether it meets the quality standards of the manufacturer.

Cutting process It is possible only because the hardness of diamond varies widely according to the direction in which one is trying to cut or grind.

A simplified round brilliant cut process includes the following stages:

• Sawing the rough stone. • Table setting where one facet is created. The table facet is then used to attach the stone into a dop

(a lapidary tool holding gemstones for cutting or polishing). • Bruting the girdle. • Blocking four main pavilion facets. • Transferring to another dop in order to rotate the stone. • Blocking four main crown facets. • Cutting and polishing all pavilion facets. • Transferring to another dop. • Cutting and polishing all crown facets.

This is just one, although a fairly common way of creating a round brilliant cut. The actual process also includes many more stages depending on the size and quality of the rough stone. For example, bigger stones are first scanned to get the three-dimensional shape, which is then used to find the optimal usage. The scanning may be repeated after each stage and bruting may be done in several steps, each bringing the girdle closer to the final shape.