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286 SYMMETRY GRADING FOR ROUND BRILLIANTS GEMS & GEMOLOGY WINTER 2011

ations during planning and cutting. Likewise, mak-ers of non-contact optical scanners have been inter-ested in guidelines for how measurable symmetryparameters affect the GIA symmetry grade. Thegrade boundaries presented here offer a substantiveestimate of the symmetry grade for any round bril-liant cut diamond.

In GIA’s laboratory, polished diamonds are mea-sured with a non-contact optical scanner early in thegrading process. Later, polish and symmetry are eval-uated visually at 10× magnification, using a standardprocedure. As described in Gillen et al. (2005), specif-ic parameter- and facet-related features are consid-ered in grading symmetry. This article presentsnumerical grade limits for 10 important symmetryparameters that can be measured accurately enoughto support visual symmetry grading. Although mea-sured values have been available to graders as a guidefor several years, beginning in 2012 GIA will usemeasured values and apply these boundary limitsstrictly when grading symmetry for round brilliantcut diamonds. Facet-related symmetry features, andthe manner in which multiple features combine,may also affect the symmetry grade, and theseaspects will continue to be evaluated visually, asthey are presently beyond reproducible instrumentmeasurement.

Compared to visual assessment, instrumentalmeasurements provide a more consistent way ofestablishing a symmetry grade, especially when a dia-mond has very subtle symmetry deviations. Figure 1shows a diamond with several symmetry flaws: awavy and uneven girdle (resulting in an unevencrown height), a table not parallel to the girdle, anduneven bezel facets. In the past, the only means of

Since 2006, GIA has used certain proportion mea-surements obtained with non-contact opticalscanners to grade the cut of round brilliant dia-

monds. Improvements in the operation and accuracyof these instruments now enable us to also measuresome symmetry parameters during the grading pro-cess. Although both Excellent and Very Good sym-metry grades meet GIA’s criteria for an Excellent cutgrade (Moses et al., 2004), there is a premium forwhat the trade calls a “triple Excellent”: anExcellent grade for cut, polish, and symmetry.Therefore, many diamond manufacturers would liketo be able to predict GIA symmetry grades frommeasurement data, so they can apply these consider-

See end of article for About the Authors.GEMS & GEMOLOGY, Vol. 47, No. 4, pp. 286–295, http://dx.doi.org/10.5741.GEMS.47.4.286.© 2011 Gemological Institute of America

Ron H. Geurts, Ilene M. Reinitz, Troy Blodgett, and Al M. Gilbertson

Grade boundary limits are presented for 10 symmetry parameters of the round brilliant cutdiamond. Starting in early 2012, these valueswill be used to support and constrain visual symmetry grading on GIA diamond reports. For manufacturers, the boundaries provide useful predictions of symmetry grades. Othersymmetry features of faceted diamonds will continue to be evaluated visually.

GIA’S SYMMETRY GRADING BOUNDARIESFOR ROUND BRILLIANT CUT DIAMONDS

SYMMETRY GRADING FOR ROUND BRILLIANTS GEMS & GEMOLOGY WINTER 2011 287

determining this diamond’s symmetry grade wouldhave been the judgment and experience of the grader.Quantifying these features by instrumental measure-ment provides a more consistent basis for symmetrygrading, and gives cutters the details needed toimprove their diamonds’ symmetry.

BACKGROUNDThe repeated measurement of any attribute, such asweight or size, is accompanied by a certain degree ofuncertainty. For example, the repeated measurementof a diamond’s weight, or its total depth, yieldsresults that vary slightly within a certain range. Forthe most accurate results, the measured value itselfand the variation in repeated measurements—theuncertainty of that value—are both important.

The U.S. National Institute of Standards andTech nology (NIST) notes that “a measurement resultis complete only when accompanied by a quantita-tive statement of its uncertainty” (“Uncertainty ofmeasurement results,” 1998). Whatever the tool ormethod, measurement results fall within a certainallowable range of values—the tolerance. For our pur-poses, the tolerance of a measuring device describesits contribution to the overall uncertainty of the mea-sured values (GIA Research, 2005).

Statistical examination of repeated independentmeasurements provides one way to estimate theiruncertainty. The distribution of these measurementsalso reveals information about reproducibility anddefensible precision. For example, a device might

measure crown angle to three decimal places, but ifrepeated measurements demonstrate an uncertaintyin the first decimal place, the two additional digitsoffer no meaningful precision (Reinitz et al., 2005).

Even detailed knowledge of the uncertainty doesnot tell us whether measurements are accurate.Accuracy can only be determined relative to the mea-surement of a known standard, such as an objectwith NIST-traceable values and reported uncertain-ties. Box A describes some basic metrology concepts,including measurement uncertainty.

The proportion values used to determine a dia-mond’s cut grade are normally the average of eightmeasurements; these are not greatly affected by a sin-gle outlying value. In contrast, symmetry parametersexamine the range (the largest and smallest) of thosevalues, and they are much more affected by a singlepoor measurement. This means a higher level of con-fidence in the reproducibility and accuracy of eachmeasured value is needed to predict a symmetrygrade, or to reinforce visual grading. GIA hasachieved this confidence through advances in thedevices used to measure polished diamonds, coupledwith efforts to ensure the diamonds are thoroughlycleaned. For example, suppose eight crown angles areindividually measured at 34.1°, 34.5°, 34.9°, 35.3°,34.2°, 34.3°, 34.0°, and 34.1°. The average is34.43°, and the difference in values (maximumminus minimum) is 1.3°. A second set of measure-ments yields values of 34.1°, 34.5°, 34.5°, 34.8°, 34.2°,34.3°, 34.0°, and 34.1°. The second average is 34.31°,

Figure 1. This 0.69 ctstandard round bril-liant cut diamonddisplays severalobvious symmetryfeatures that can bequantified. Photo byRobert Weldon.


only 0.12° below the previous average. But the differ-ence in values is now 0.8°, considerably smaller thanthe 1.3° from the first set of measurements. In otherwords, the average changes less than the difference invalues when one or two of the eight values is marredby dirt or some other measuring problem not specifi-cally related to that particular diamond.

Higher-quality measurements have a smalleruncertainty, but even the best measuring systemshave some tolerance for each parameter. Box Ashows an example of measurement uncertainty atthe border between Very Good and Excellent. Evenmeasurements of clean diamonds made on devices ofproven accuracy and precision can produce one ormore values that fall within tolerance of a symmetry

grade boundary. Multiple measurements, on onedevice or on different devices, can yield slightly dif-ferent results. All devices have a margin of error(within the tolerance of the device) that could yieldtwo different grades when one or more parametersare near a border. Since no measurement is exact,prudent cut planning acknowledges measuring toler-ances and avoids placing parameters too close to theborders.

MEASURABLE SYMMETRY PARAMETERS ANDADDITIONAL FACTORSTen symmetry parameters are illustrated in figure 2.GIA has developed procedures to measure these

Taking several independent measurements of thesame characteristic illustrates the difference

between precision and accuracy, as shown in figureA-1. Accuracy refers to how close the measured val-ues are to the reference value, shown here as thecenter of the target. Precision refers to how close thevalues are to each other, and in practice this affectshow many significant figures should be used whenreporting the measurement.

When the difference between two measured val-ues is less than or equal to the measurement uncer-tainty, the values are within tolerance of each other,and by definition not readily distinguishable fromone another. Figure A-2 shows six measurements ofthe total depth of one round brilliant, each with anuncertainty of ±0.015 mm. The average value ofthose measurements is 5.015 mm. Trial 4, with avalue of 5.00 mm, is just within tolerance of thataverage. Trial 6, with a value of 5.04 mm, is notwithin tolerance of the average. This is described asan outlying value.

Most gemologically important parameters for theround brilliant cut diamond, such as the crown orpavilion angle, represent averages rather than singlemeasurements. In metrology, averages of multiplemeasurements are used to reduce measurementuncertainty. But a quantity such as average crownangle is calculated from eight values obtained fromdifferent facets, rather than eight measurements ofthe same facet. As a result, this average has its ownuncertainty that is no smaller than the uncertainties

of the eight individual values. In a round brilliant oflower symmetry, the eight crown angle values may varyby several degrees. The uncertainty of a symmetryassessment for such variation among the crown angles

BOX A: BASIC MEASURING CONCEPTS

Not AccurateNot Precise

AccurateNot Precise

Not AccuratePrecise

AccuratePrecise

Figure A-1. A measurement is accurate when itagrees with an independently obtained referencevalue (here, the center of the bull’s-eye). Measure-ments are precise when they can be reproduced withsmall uncertainties. The ideal situation is to havemeasurements that are both accurate and precise.

parameters with sufficient accuracy to determinethe symmetry grade of round brilliant cut diamonds.Additional parameters have been identified, such asthe symmetry of the star facets and the upper andlower girdle facets, but numerical boundaries forthese are still under review.

The 10 symmetry parameters are calculated asfollows:

1. Out-of-round: the difference between the maxi-mum and minimum diameter, as a percentageof the average diameter

2. Table off-center: the direct distance betweenthe table center and the outline center projectedinto the table plane, as a percentage of the aver-age diameter


NEED TO KNOW

• Starting in early 2012, GIA will apply boundarylimits for 10 symmetry parameters measured bynon-contact optical scanners when grading thesymmetry of round brilliant cut diamonds.

• Additional measurable parameters, aspects arisingfrom combinations of these parameters, and facet-related symmetry variations will continue to be assessed visually.

• Manufacturers should strive to attain values that are20% lower than the symmetry boundary limits, toaccount for measurement uncertainty and featuresthat may combine to lower the symmetry grade.

COMPARING VALUES WITH UNCERTAINTIES

4.98

4.99

5.00

5.01

5.02

5.03

5.04

5.05

5.06

1 3 4 5 6

TRIAL

TOTA

L D

EPTH

(m

m)

2

UNCERTAINTY VS. A SYMMETRY BOUNDARY

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1 3 4 5

REPEATED MEASUREMENTSOF SAME STONE

CR

OW

N A

NG

LEV

AR

IATI

ON

(˚)

2

Figure A-3. A round brilliant measured five timesyields crown angle–variation values with uncertain-ties that cross the symmetry grade limit for thisparameter (1.2°). Although the third measurement of1.3° would indicate Very Good symmetry, the mostreproducible value—the one most often obtained—iswithin the limits for Excellent.

Figure A-2. These total-depth measurements are shownwith error bars that represent measurement uncertainty.These bars overlap the average value of 5.015 mm for thefirst five trials, but not the sixth. It is important to recog-nize the distinction between (1) measurements withintolerance of each other, and (2) measurements that clear-ly differ from each other beyond the tolerance. If the errorbars overlap each other, the measurements can be consid-ered the same; if they do not overlap, the measurementsare different.

is also no smaller than the individual uncertainties. The uncertainty associated with a measured

value can be thought of as a “bubble” around it.Overlap among these bubbles in a group of measure-ments indicates agreement with each other. A fixedboundary, such as a limit for symmetry grading, cancut through such uncertainty bubbles, separating agroup of measurements that agree with each other

into two differing results. Figure A-3 shows such anexample, where all five measurements are within tol-erance of each other, but one generates a symmetrygrade of Very Good, based on this one parameter,while the other four would score in the Excellentrange. From basic metrological principles, if the mea-suring device is sound, the more reproducible valueis the correct one.


QUANTIFIED SYMMETRY FEATURES

Out-of-round: deviation from the circular shape of a round diamond

Table off-center: deviation of the table from the central position on the crown

Culet off-center: deviation of the culet from the central position on the pavilion

Table/culet alignment: displacement of the table facet and culet in opposite directions

Table size variation: differing table size measurments indicating non-octagonal table

Girdle thickness variation: variation of the girdle thickness at bezel positions

Crown height variation: differing crown height measurements indicating a wavy girdle or table/girdle not parallel

Crown angle variation: crown angles are unequal; typically related to table off-center

Pavilion depth variation: differing pavilion depth measurements indicating a wavy girdle

Pavilion angle variation: pavilion angles are unequal; typically related to culet off-center

Figure 2. These 10 symmetry features can be measured reliably enough by non-contact optical scanners to determine the symmetry grade of round brilliant cut diamonds.


Figure 3. Three vertical lengths (A1–A3)in this close-up of the 0.69 ct diamond infigure 1 illustrate girdle thickness differ-ences. Region B (green circle) showswhere the facet edges of the upper andlower girdle do not meet (crown andpavilion misalignment). Region C (yel-low circle) shows the junction wherethree facets fail to meet (pointingfault). Photo by Robert Weldon.

3. Culet off-center: the direct distance betweenthe culet center and the outline center project-ed into any horizontal plane such as the tableplane, as a percentage of the average diameter

4. Table/culet alignment: the direct distancebetween the table center and the culet centerprojected into the table plane, as a percentage ofthe average diameter

5. Crown height variation: the difference betweenthe maximum and minimum crown height val-ues, as a percentage of the average diameter

6. Crown angle variation: the difference betweenthe maximum and minimum crown angle val-ues, in degrees

7. Pavilion depth variation: the differencebetween the maximum and minimum paviliondepth values, as a percentage of the averagediameter

8. Pavilion angle variation: the difference betweenthe maximum and minimum pavilion anglevalues, in degrees

9. Girdle thickness variation: the differencebetween the maximum and minimum girdlethickness values, as a percentage of the averagediameter, measured at the bezel-main junctions(see also features A1–A3 in figure 3)

10. Table size variation: the difference between themaximum and minimum table size values, as apercentage of the average diameter

Because the facets of a round brilliant are con-nected to each other, these symmetry features fre-quently occur in combination. All of the symmetryfeatures combine to produce a general face-up visualimpression, so the symmetry grade is established bylooking at the face-up diamond. Depending onwhere they occur, and how they combine, differentsymmetry features can visually amplify or compen-sate for one another, as discussed in box B. This

interaction plays a large role in determining theoverall symmetry grade for round brilliants withlower symmetry. But for those with high symmetry,the magnitude of a single feature may dominate theevaluation.

Facet-related symmetry features also play a role indetermining the symmetry grade (e.g., figure 3, fea-tures B and C), but they are not part of the grading pro-cedure described here. A full description of facet-relat-ed symmetry features can be found in Blodgett et al.(2009). Open or short facets (non-pointing), misalign-ment between the bezels and pavilion mains, andprominent naturals or extra facets are readilyobserved, but they may occur independently of the 10measurable symmetry parameters listed above.Misshapen or uneven facets usually relate to a combi-nation of the 10 parameters, but the relationships canbe complex.

RECOMMENDED SYMMETRY BOUNDARIESThe limits given below were derived from a statisti-cal comparison of measured values for the 10 param-eters and the final symmetry grades assigned to thediamonds. This comparison was repeated four timesover a period of 10 years, each time on newlyacquired data sets from several thousand diamonds.Each analysis examined several sets of limits for the10 parameters to identify robust matches with visualsymmetry grading.

Table 1 presents the ranges of allowed values forindividual symmetry features, measured in percent-age or degrees, that GIA uses to support and con-strain visual symmetry grading. The limits dividingFair from Poor symmetry are not presented herebecause of the small number of round brilliants withsuch low symmetry. Measured values should berounded to the indicated precision, if necessary,before calculating the differences. If the value for anyone parameter falls into a range associated with alower grade, the overall symmetry grade will be low-

B


T he red dashed lines in the drawings in figure B-1show the position of the table center, and the

blue dot shows the center of the stone outline.Consider two round brilliants, each with a 4% off-center table and culet (cases B and C). The measuredvalues for table and culet being off center are equal,and each feature would be easily noticed individual-ly, in profile as well as face-up. When the culet andtable are off center in the same direction (case B), thetwo symmetry features compensate for each othervisually. But when the table and culet are shifted offcenter in different directions (case C), the negativevisual impression is amplified considerably.

When uneven crown height and girdle thick-

ness are added to the off-center table and culet, thevisual difference between various combinations ofthese features becomes even more pronounced. Infigure B-2 (top), the table and culet are off center inthe same direction, and the girdle and crown heightare uneven along this same A-B axis. Arranged inthis way, these features tend to compensate eachother visually, particularly in the face-up view. Incontrast, figure B-2 (bottom) shows a table andculet that are off center in opposite directions, andthe girdle thickness and crown height are unevenin a different direction (along the G-H axis). Thiscombination amplifies the visual impression ofasymmetry.

BOX B: COMBINATIONS OF SYMMETRY FEATURES

A B C

Culet and table centered Culet and table off center4% in same direction

Culet and table off-center4% in opposite direction

Figure B-1. A culet andtable that are off center in

different directions producea more asymmetrical

appearance (case C) thanwhen they are off center in

the same direction (case B).Note that the degree ofasymmetry is extreme,down to the Fair range.

ered accordingly. Combinations of symmetry fea-tures, as well as facet-related features that are notmeasured, will still be evaluated visually, whichmay also contribute to a lower symmetry grade.

For example, if nine of the parameters are withinthe Excellent range but the table is off-center by0.7%, the best possible symmetry grade is VeryGood. If all 10 parameters are within the Excellentrange, the expected symmetry grade would beExcellent. But consider a round brilliant that is outof round by 0.7%, with crown angle variation of 1.1°and girdle thickness variation of 1.1%. Even thoughall three parameters are within the limits for

TABLE 1. Limits used by GIA to grade the symmetry of round brilliant cut diamonds.

Parameter Excellent Very Good Good

Out-of-round (%) 0–0.9 1.0–1.8 1.9–3.6Table off-center (%) 0–0.6 0.7–1.2 1.3–2.4Culet off-center (%) 0–0.6 0.7–1.2 1.3–2.4Table/culet alignment (%) 0–0.9 1.0–1.8 1.9–3.6Crown height variation (%) 0–1.2 1.3–2.4 2.5–4.8Crown angle variation (°) 0–1.2 1.3–2.4 2.5–4.8Pavilion depth variation (%) 0–1.2 1.3–2.4 2.5–4.8Pavilion angle variation (°) 0–0.9 1.0–1.8 1.9–3.6Girdle thickness variation (%) 0–1.2 1.3–2.4 2.5–4.8Table size variation (%) 0–1.2 1.3–2.4 2.5–4.8


Figure B-2. These two round brilliants have multiple measurable symmetry faults that limit them tono better than a Good symmetry grade. Although both stones have equal culet off-center values, theappearance of overall symmetry is different because of the relative placement of the various symmetryfaults. The green crosshair indicates the center of the outline, blue is the center of the table, and reddenotes the center of the culet. When the faults are aligned, the asymmetry appears less pronounced(top). By comparison, when symmetry faults occur in different directions, the visual impression ofasymmetry is amplified (bottom). In either combination, these displacements are considerably moresubtle than those shown in figure B-1.

Excellent, the combination of these three symmetryfeatures (and any others found on the diamond) mayresult in either an Excellent or a Very Good symme-try grade, depending on the visual assessment.

Because every measurement contains uncertain-ty, and symmetry features may combine to lowerthe symmetry grade, we recommend a “safety mar-gin” for the trade to use in estimating the symmetrygrade. Accordingly, the values shown in table 2 are20% lower than those in table 1. When the valuesfor all 10 parameters fall within these narrower rec-ommended borders, there is a strong likelihood thatthe visual symmetry assessment will agree with the

TABLE 2. Recommended limits for estimating the symmetry grade of round brilliant cut diamonds.

Parameter Excellent Very Good Good

Out-of-round (%) 0–0.7 0.8–1.4 1.5–2.8Table off-center (%) 0–0.5 0.6–1.0 1.1–1.9Culet off-center (%) 0–0.5 0.6–1.0 1.1–1.9Table/culet alignment (%) 0–0.7 0.8–1.4 1.5–2.8Crown height variation (%) 0–1.0 1.1–2.0 2.1–3.9Crown angle variation (°) 0–1.0 1.1–2.0 2.1–3.9Pavilion depth variation (%) 0–1.0 1.1–2.0 2.1–3.9Pavilion angle variation (°) 0–0.7 0.8–1.4 1.5–2.8Girdle thickness variation (%) 0–1.0 1.1–2.0 2.1–3.9Table size variation (%) 0–1.0 1.1–2.0 2.1–3.9

measurement. Within these recommended limits, itis unlikely that a combination of measurable sym-metry features would lead to a lower symmetrygrade. Note that the second example in the previousparagraph exceeds two of these recommended limits.

The boundary values presented for these 10 sym-metry features are most useful along theExcellent–Very Good symmetry border, where a sin-gle feature often dominates the final grade determi-nation. These individual parameter limits are alsorelevant for the border between Very Good andGood. When symmetry problems become severe,

though, it is more likely that multiple symmetryfeatures will limit the grade, because the interac-tions among symmetry factors become more pro-nounced (again, see box B). Because combinations ofminor symmetry features can create a significantvisual impact, the limits in the tables must beviewed only as a guide.

DISCUSSIONDuring the analysis of laboratory grading results, weobserved some variation in how strictly symmetrywas evaluated by our graders, particularly for mea-


A

C

B

Figure 4. These threeround brilliants each dis-play a combination ofsymmetry faults. (A) Thetable of this 1.00 ct dia-mond (Fair symmetry) isnot an octagon (6.1%table size variation, asshown by the blue andyellow lines) and the tableis off-center by 2.5%. Theasymmetry of the table isassociated with crownangle variations anduneven bezels (markedred). (B) The culet of this0.83 ct diamond (Fairsymmetry) is off-center by2.9% (red dot). The tableis also off-center in anopposing direction (greendot), yielding a value fortable/culet alignment of3.4%. These symmetryfaults are associated withuneven bezels (markedred) and pavilion mains.Unlike the diamond in A,the nearly equal quad-rants defined by the yel-low lines show that thetable is octagonal. (C) Inthis 0.69 ct diamond (alsoshown in figures 1 and 3;Good symmetry), the gir-dle is wavy and not paral-lel to the table. Photos byGIA (A and B) and RobertWeldon (C).


REFERENCESBlodgett T., Geurts R., Gilbertson A., Lucas A., Pay D., Reinitz I.,

Shigley J., Yantzer K., Zink C. (2009) Finish, culet size and gir-dle thickness; Categories of the GIA Diamond Cut GradingSystem. www.gia.edu/diamondcut/pdf/poster_finish_culet_girdle_highres.pdf [date accessed: June 14, 2011].

GIA Research (2005) Measurement tolerances: Accuracy and pre-cision in the gem industry. Rapaport Diamond Report, Vol.28, No. 13, pp. 183–185, www.gia.edu/diamondcut/pdf/4_05_RDR_pg183_185.pdf.

Gillen D.B., Lanzl B.F., Yantzer P.M. (2005) Polish and symmetry.Rapaport Diamond Report, Vol. 28, No. 39, pp. 80–87,www.gia.edu/diamondcut/pdf/polish_and_symmetry.pdf.

Moses T.M., Johnson M.L., Green B., Blodgett T., Cino K., GeurtsR.H., Gilbertson A.M., Hemphill T.S., King J.M., Kornylak L.,Reinitz I.M., Shigley J.E. (2004) A foundation for grading the over-all cut quality of round brilliant cut diamonds. G&G, Vol. 40,No. 3, pp. 202–228, http://dx.doi.org/10.5741.GEMS.40.4.202.

Reinitz I., Yantzer K., Johnson M., Blodgett T., Geurts R.,Gilbertson A. (2005) Proportion measurement: Tolerances forthe GIA Diamond Cut Grading System. Rapaport DiamondReport, Vol. 28, No. 30, pp. 34–39, www.gia.edu/diamondcut/pdf/0805_pg34_39.pdf.

Uncertainty of measurement results (1998) The NIST Referenceon Constants and Uncertainty. http://physics.nist.gov/cuu/Uncertainty/international1.html [date accessed: June 14, 2011].

ABOUT THE AUTHORSMr. Geurts is a manager of research and development at GIAin Antwerp. Dr. Reinitz is a project manager at GIA in NewYork. Dr. Blodgett is a research scientist, and Mr. Gilbertson aresearch associate, at GIA in Carlsbad.

sured features near the border between Excellent andVery Good. A common set of fixed numerical limitsfor these parameters can only improve the consisten-cy of symmetry grading for such stones. Diamondswith at least one parameter beyond the limits shownin table 1 will receive the lower symmetry grade.Symmetry features not captured by these 10 param-eters will continue to be evaluated visually. If theseadditional facet-related features are sufficientlyprominent—an extra facet polished at the corner ofthe table, for instance—they will reduce the sym-metry grade even if all measured parameters fallwithin the narrower limits in table 2. Visual sym-metry observations cannot raise a symmetry grade,but they can reveal instances when a cleaner, morecorrect measurement of the diamond is needed.

Measured values can be of great help for dia-monds with multiple symmetry faults, such as thethree shown in figure 4. In such cases, some of thesymmetry features are more easily noticed visual-ly, while others are captured more accurately bymeasurement. In figure 4A, the asymmetry of thetable leads to variation in crown angles anduneven bezel facets. In other cases, similar faultswith the table might be associated with a wavygirdle that takes up the uneven aspects of the crownand allows little variation in the crown angles. Underboth sets of circumstances, the uneven bezels are aprominent feature that does not describe the underly-ing symmetry faults as clearly as the measured valuesfor crown angle variation, crown height variation, andgirdle thickness variation.

In figure 4B, the off-center culet and table leadto uneven bezels and pavilion mains. The displace-ment between the table center and the culet

emphasizes the visual impact of the off-centerculet (again, see box B), but the measured values—that is, Good for table-culet alignment, but Fair fortable off-center—provide a context for evaluatingthe severity of the combination. In figure 4C, themost prominent symmetry fault is displayed forthe diamond shown in figure 3. The table and gir-dle are not parallel, a fault that is more severe thanthe uneven girdle thickness or the facet-relatedsymmetry features.

CONCLUSIONMeasurement is a process full of inherent uncertain-ties, but GIA’s efforts to achieve smaller uncertain-ties have been successful. Starting in early 2012, themeasurable values presented in table 1 will be usedto attain greater consistency than is possiblethrough visual assessment alone. Additional mea-surable parameters, aspects arising from combina-tions of these parameters, and facet-related symme-try variations will continue to be assessed visually.A more restrictive set of limits is recommended formanufacturers, to help ensure that the final symme-try grade will not be undermined by combinationeffects or measuring tolerances.

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