verification of the authenticity and classification of ......4 verification of the authenticity and...

Verification of the Authenticity

and Classification of Gemstones

Using XRF

Introduction

For thousands of years, pearls, emeralds,

rubies and sapphires, as well as other precious

jewelry and mineral stones such as alexandrites,

aquamarines, etc., have been greatly valued due

to their rarity, finite nature and beauty. Nowadays

they are mainly used in the jewelry industry,

science and gemology.

A WHITE PAPER FROMSPECTRO ANALYTICAL INSTRUMENTS

It has even become technically possible to

artificially manufacture gemstones. These artificial

gemstones are usually of significantly lesser value

than natural gemstones, but they are scarcely

different in color and shape. In addition, natural

gemstones and pearls vary in value depending on

their origin due to their color, color properties when

exposed to light and their prevalence in nature.

When results matter

2 Verification of the Authenticity and Classification of Gemstones Using XRF

METHODOLOGYSo how is such a differentiation of gem-

stones, whether synthetic or real or whether

salt or freshwater pearl, possible? An ele-

mental analysis by means of energy disper-

sive X-ray fluorescence analysis (ED-XRF)

provides information on the elemental com-

position of gemstones and pearls and is also

completely non-destructive. The sample is

not damaged and can also be directly ana-

lyzed without having to prepare the sample.

The analysis can be performed with various

spectrometers, such as the SPECTROCUBE,

SPECTRO MIDEX or SPECTROSCOUT, the

latter of which can also be used on site, the

SPECTROCUBE and the SPECTRO MIDEX

can also be used for the analysis of precious

metal alloys. For the analysis of pure unpro-

cessed stones and pearls, devices in which

the samples to be examined can be placed

directly on the surface and analyzed from

below have proven to be advantageous.

For the analysis of jewelry and thus complex

geometries, analysis from above is more

advantageous as it is done by the SPECTRO

MIDEX. This device has an integrated video

system and supports the positioning and

documentation of the respective measuring

point and can also analyze the precious met-

al quickly and precisely.

Figure 1: The SPECTRO MIDEX offers an

integrated video system

3Verification of the Authenticity and Classification of Gemstones Using XRF

Such measurements allow to differentiate

between artificial and natural gemstones.

Figure 2 shows a comparison spectrum of an

artificial and a natural sapphire.

Artificial gemstones can be distinguished

from natural gemstones based on their col-

oring elements. In the above spectrum, the

blue color of the synthetic sapphire has been

created by adding cobalt oxide. However, no

cobalt oxide is present in the natural gem-

stone. Instead, the color originates from iron

oxide.

Figure 3 shows a comparison spectrum of a

natural and an artificial ruby. Traces of addi-

tional elements such as gallium and bismuth

are detectable in natural gemstones, which

are not present in synthetic gemstones.

These are spectrally purer, so they contain

hardly any additional trace elements.

Figure 2: Comparison spectrum of a synthetic (red spectrum) and a natural sapphire

(blue spectrum)

Figure 3: Comparison spectrum of a synthetic (red spectrum) and a natural ruby

(black spectrum)

Energy [keV]

Energy [keV]

Impu

lses

[x10

4 ]Im

puls

es [x

104 ]


It is also possible to differentiate based on

main components, i.e. whether a ruby or

sapphire is really corundum (Al2O3) or just

colored glass (SiO2). The spectrum in figure

4 shows the corundum ruby and sapphire, as

well as an emerald with clearly different sig-

nals of the main components Al and Si.

The contents of detectable elements range

from about 10 ppm to a percentage range.

Generally, all elements from Na to U can be

detected but those elements listed in table 1

are typically found in gemstones.

Elements Range of concentration Elements Range of concentration

Mg Matrix [%] Zn 10-3000 ppm

Al Matrix [%] Ga 10-600 ppm

Si Matrix [%] Rb 10-300 ppm

K 10-3000 ppm Sr 10-500 ppm

Ca 10-35000 ppm Zr 10-600 ppm

Sc 10-100 ppm Nb 10-20 ppm

Ti 10-500 ppm Mo 10-100ppm

V 100-2000 ppm Sn 10-200 ppm

Cr 10-18000 ppm Sb 10-100 ppm

Mn 50-13000 ppm Cs 50-2000 ppm

Fe 10-100000 ppm Ta 10-100 ppm

Co 10-100 ppm W 10-50 ppm

Cu 10-15000 ppm Pb 10-2000 ppm

Ni 10-500 ppm Bi 10-10000 ppm

Figure 4: Spectra of ruby, sapphire and emerald

Table 1: Typical elements in gemstones and

their ranges of concentration

Energy [keV]

Impu

lses

[x10

5 ]


The following four figures show correlation

curves of relevant elements. A good correla-

tion between reference concentration and

analytical result can be seen.

Figures 5-8: Correlations of Cr, Fe, Bi and Mn in

emeralds and tourmalines

R² = 0,9927

0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6

Anal

ysis

resu

lt [%

]

Reference concentration [%]

Correlation of Cr in Emerald

R² = 0,9982

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

0 0,5 1 1,5 2 2,5 3 3,5 4

Anal

ysis

resu

lt [%

]


Correlation of Fe in Emerald

R² = 0,9967

0,15

0,2

0,25

0,3

0,35

0,4

0,45

0,5

0,15 0,2 0,25 0,3 0,35 0,4 0,45 0,5

Anal

ysis

resu

lt [%

]


Correlation of Bi in Tourmaline

R² = 0,9903

0

0,2

0,4

0,6

0,8

1

1,2

1,4

0 0,2 0,4 0,6 0,8 1 1,2 1,4

Anal

ysis

resu

lt [%

]


Correlation of Mn in Tourmaline


If you want to determine the origin of gem-

stones, you will either require knowledge

about the elemental composition typical for

the region or you will need to build a data-

base.

Figure 9: Elemental distribution of chromium to iron in

emeralds [Source: Guild]

Figure 10: Elemental distribution of iron to chromium in

rubies [Source: Guild]

Figures 9 and 10 show the correlations of

iron to chromium in emeralds and rubies in

different regions of the world. For example,

an emerald from Colombia has a relatively

low iron content compared to emeralds from

Zambia. For rubies, the countries Burma,

Mozambique and Tanzania are shown as

clusters.


PEARLS

What for gemstones is similarly valid for

pearls. Pearls can be cultured, but a saltwater

pearl is much more valuable than a freshwa-

ter pearl due to the effort required to cultivate

it, and it looks very similar.

Determining whether it is a fresh or salt water

pearl can be done with an XRF instrument by

analyzing the manganese content, because

the matrix in both kinds is calcium carbon-

ate (CaCO3) and thus identical. If a pearl is

cultured in fresh water, more manganese is

stored in it than in a saltwater pearl. Fresh

water contains a relatively high amount of

manganese (5 ppb) compared to salt water of

the oceans (0.03 - 2 ppb). This difference can

be detected with ED-XRF as the spectrum in

Figure 11 shows, which is a comparison of

an analysis of a fresh and a salt water pearl.

Figure 11: Comparison on an analysis of a freshwater and a

saltwater pearl

Figure 12: Above: freshwater pearls, below: saltwater pearls

Energy [keV]

Impu

lses

[x10

2 ]


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Summary:

Through the purity and the presence of significant elements, the authenticity and even origin of the gemstones can

be inferred. This helps, for example, in determining the value. Background knowledge about the presence of trace

elements typical for the respective region is required, especially when deciding of the gemstone's origin. In pearls, this

element is essentially manganese, which makes the difference between salt and freshwater pearls. Energy dispersive

X-ray fluorescence analysis (ED-XRF) can be used for all these tasks and can thus allow us to obtain certainty regarding

information on gemstones and pearls and ultimately differentiate natural from synthetic gemstones.

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