GIA's Symmetry Grading Boundaries for Round Brilliant Cut Diamonds

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 symmetry

parameters affect the GIA symmetry grade. The

grade boundaries presented here offer a substantive

estimate 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 the

grading process. Later, polish and symmetry are eval-

uated visually at 10× magnification, using a standard

procedure. As described in Gillen et al. (2005), specif-

ic parameter- and facet-related features are consid-

ered in grading symmetry. This article presents

numerical grade limits for 10 important symmetry

parameters that can be measured accurately enough

to support visual symmetry grading. Although mea-

sured values have been available to graders as a guide

for several years, beginning in 2012 GIA will use

measured values and apply these boundary limits

strictly when grading symmetry for round brilliant

cut diamonds. Facet-related symmetry features, and

the manner in which multiple features combine,

may also affect the symmetry grade, and these

aspects will continue to be evaluated visually, as

they are presently beyond reproducible instrument

measurement.

Compared to visual assessment, instrumental

measurements provide a more consistent way of

establishing a symmetry grade, especially when a dia-

mond has very subtle symmetry deviations. Figure 1

shows a diamond with several symmetry flaws: a

wavy and uneven girdle (resulting in an uneven

crown height), a table not parallel to the girdle, and

uneven bezel facets. In the past, the only means of

ince 2006, GIA has used certain proportion mea-

surements obtained with non-contact optical

scanners to grade the cut of round brilliant dia-

monds. Improvements in the operation and accuracy

of these instruments now enable us to also measure

some symmetry parameters during the grading pro-

cess. Although both Excellent and Very Good sym-

metry grades meet GIA’s criteria for an Excellent cut

grade (Moses et al., 2004), there is a premium for

what the trade calls a “triple Excellent”: an

Excellent grade for cut, polish, and symmetry.

Therefore, many diamond manufacturers would like

to be able to predict GIA symmetry grades from

measurement 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.

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 cut

diamond. Starting in early 2012, these values

will be used to support and constrain visual

symmetry grading on GIA diamond reports.

For manufacturers, the boundaries provide

useful predictions of symmetry grades. Other

symmetry features of faceted diamonds will

continue to be evaluated visually.

GIA’S SYMMETRY GRADING BOUNDARIES

FOR ROUND BRILLIANT CUT DIAMONDS

SYMMETRY GRADING FOR ROUND BRILLIANTS GEMS & GEMOLOGY WINTER 2011 287

determining this diamond’s symmetry grade would

have been the judgment and experience of the grader.

Quantifying these features by instrumental measure-

ment provides a more consistent basis for symmetry

grading, and gives cutters the details needed to

improve their diamonds’ symmetry.

BACKGROUND

The repeated measurement of any attribute, such as

weight or size, is accompanied by a certain degree of

uncertainty. For example, the repeated measurement

of a diamond’s weight, or its total depth, yields

results that vary slightly within a certain range. For

the most accurate results, the measured value itself

and the variation in repeated measurements—the

uncertainty of that value—are both important.

The U.S. National Institute of Standards and

Tech nology (NIST) notes that “a measurement result

is complete only when accompanied by a quantita-

tive statement of its uncertainty” (“Uncertainty of

measurement results,” 1998). Whatever the tool or

method, measurement results fall within a certain

allowable range of values—the tolerance. For our pur-

poses, the tolerance of a measuring device describes

its contribution to the overall uncertainty of the mea-

sured values (GIA Research, 2005).

Statistical examination of repeated independent

measurements provides one way to estimate their

uncertainty. The distribution of these measurements

also reveals information about reproducibility and

defensible precision. For example, a device might

measure crown angle to three decimal places, but if

repeated measurements demonstrate an uncertainty

in the first decimal place, the two additional digits

offer no meaningful precision (Reinitz et al., 2005).

Even detailed knowledge of the uncertainty does

not tell us whether measurements are accurate.

Accuracy can only be determined relative to the mea-

surement of a known standard, such as an object

with 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 eight

measurements; these are not greatly affected by a sin-

gle outlying value. In contrast, symmetry parameters

examine the range (the largest and smallest) of those

values, and they are much more affected by a single

poor measurement. This means a higher level of con-

fidence in the reproducibility and accuracy of each

measured value is needed to predict a symmetry

grade, or to reinforce visual grading. GIA has

achieved this confidence through advances in the

devices used to measure polished diamonds, coupled

with efforts to ensure the diamonds are thoroughly

cleaned. For example, suppose eight crown angles are

individually measured at 34.1°, 34.5°, 34.9°, 35.3°,

34.2°, 34.3°, 34.0°, and 34.1°. The average is

34.43°, and the difference in values (maximum

minus 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 ct

standard round bril-

liant cut diamond

displays several

obvious symmetry

features that can be

quantified. Photo by

Robert Weldon.

288 SYMMETRY GRADING FOR ROUND BRILLIANTS GEMS & GEMOLOGY WINTER 2011

only 0.12° below the previous average. But the differ-

ence in values is now 0.8°, considerably smaller than

the 1.3° from the first set of measurements. In other

words, the average changes less than the difference in

values when one or two of the eight values is marred

by dirt or some other measuring problem not specifi-

cally related to that particular diamond.

Higher-quality measurements have a smaller

uncertainty, but even the best measuring systems

have some tolerance for each parameter. Box A

shows an example of measurement uncertainty at

the border between Very Good and Excellent. Even

measurements of clean diamonds made on devices of

proven accuracy and precision can produce one or

more values that fall within tolerance of a symmetry

grade boundary. Multiple measurements, on one

device 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 yield

two different grades when one or more parameters

are near a border. Since no measurement is exact,

prudent cut planning acknowledges measuring toler-

ances and avoids placing parameters too close to the

borders.

MEASURABLE SYMMETRY PARAMETERS AND

ADDITIONAL FACTORS

Ten symmetry parameters are illustrated in figure 2.

GIA has developed procedures to measure these

aking several independent measurements of the

same characteristic illustrates the difference

between precision and accuracy, as shown in figure

A-1. Accuracy refers to how close the measured val-

ues are to the reference value, shown here as the

center of the target. Precision refers to how close the

values are to each other, and in practice this affects

how many significant figures should be used when

reporting 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 from

one another. Figure A-2 shows six measurements of

the total depth of one round brilliant, each with an

uncertainty of

0.015 mm. The average value of

those measurements is 5.015 mm. Trial 4, with a

value of 5.00 mm, is just within tolerance of that

average. Trial 6, with a value of 5.04 mm, is not

within tolerance of the average. This is described as

an outlying value.

Most gemologically important parameters for the

round brilliant cut diamond, such as the crown or

pavilion angle, represent averages rather than single

measurements. In metrology, averages of multiple

measurements are used to reduce measurement

uncertainty. But a quantity such as average crown

angle is calculated from eight values obtained from

different facets, rather than eight measurements of

the same facet. As a result, this average has its own

uncertainty that is no smaller than the uncertainties

of the eight individual values. In a round brilliant of

lower symmetry, the eight crown angle values may vary

by several degrees. The uncertainty of a symmetry

assessment for such variation among the crown angles

BOX A: BASIC MEASURING CONCEPTS

Not Accurate

Not Precise

Accurate

Not Precise

Not Accurate

Precise

Accurate

Precise

Figure A-1. A measurement is accurate when it

agrees with an independently obtained reference

value (here, the center of the bull’s-eye). Measure-

ments are precise when they can be reproduced with

small uncertainties. The ideal situation is to have

measurements that are both accurate and precise.

parameters with sufficient accuracy to determine

the symmetry grade of round brilliant cut diamonds.

Additional parameters have been identified, such as

the symmetry of the star facets and the upper and

lower girdle facets, but numerical boundaries for

these are still under review.

The 10 symmetry parameters are calculated as

follows:

1. Out-of-round: the difference between the maxi-

mum and minimum diameter, as a percentage

of the average diameter

2. Table off-center: the direct distance between

the table center and the outline center projected

into the table plane, as a percentage of the aver-

age diameter

SYMMETRY GRADING FOR ROUND BRILLIANTS GEMS & GEMOLOGY WINTER 2011 289

NEED TO KNOW

• Starting in early 2012, GIA will apply boundary

limits for 10 symmetry parameters measured by

non-contact optical scanners when grading the

symmetry of round brilliant cut diamonds.

• Additional measurable parameters, aspects arising

from combinations of these parameters, and

facet-related symmetry variations will continue to

be assessed visually.

• Manufacturers should strive to attain values that are

20% lower than the symmetry boundary limits, to

account for measurement uncertainty and features

that 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 3 456

TRIAL

TOTAL DEPTH (mm)

UNCERTAINTY VS. A SYMMETRY BOUNDARY

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1345

REPEATED MEASUREMENTS

OF SAME STONE

CROWN ANGLE

VARIATION (

)

Figure A-3. A round brilliant measured five times

yields crown angle–variation values with uncertain-

ties that cross the symmetry grade limit for this

parameter (1.2°). Although the third measurement of

1.3° would indicate Very Good symmetry, the most

reproducible value—the one most often obtained—is

within the limits for Excellent.

Figure A-2. These total-depth measurements are shown

with error bars that represent measurement uncertainty.

These bars overlap the average value of 5.015 mm for the

first five trials, but not the sixth. It is important to recog-

nize the distinction between (1) measurements within

tolerance of each other, and (2) measurements that clear-

ly differ from each other beyond the tolerance. If the error

bars overlap each other, the measurements can be consid-

ered the same; if they do not overlap, the measurements

are 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 fixed

boundary, such as a limit for symmetry grading, can

cut through such uncertainty bubbles, separating a

group of measurements that agree with each other

into two differing results. Figure A-3 shows such an

example, where all five measurements are within tol-

erance of each other, but one generates a symmetry

grade of Very Good, based on this one parameter,

while the other four would score in the Excellent

range. From basic metrological principles, if the mea-

suring device is sound, the more reproducible value

is the correct one.

290 SYMMETRY GRADING FOR ROUND BRILLIANTS GEMS & GEMOLOGY WINTER 2011

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.

SYMMETRY GRADING FOR ROUND BRILLIANTS GEMS & GEMOLOGY WINTER 2011 291

Figure 3. Three vertical lengths (A1–A3)

in this close-up of the 0.69 ct diamond in

figure 1 illustrate girdle thickness differ-

ences. Region B (green circle) shows

where the facet edges of the upper and

lower girdle do not meet (crown and

pavilion misalignment). Region C (yel-

low circle) shows the junction where

three facets fail to meet (pointing

fault). Photo by Robert Weldon.

Culet off-center: the direct distance between

the culet center and the outline center project-

ed into any horizontal plane such as the table

plane, as a percentage of the average diameter

4. Table/culet alignment: the direct distance

between the table center and the culet center

projected into the table plane, as a percentage of

the average diameter

5. Crown height variation: the difference between

the maximum and minimum crown height val-

ues, as a percentage of the average diameter

6. Crown angle variation: the difference between

the maximum and minimum crown angle val-

ues, in degrees

7. Pavilion depth variation: the difference

between the maximum and minimum pavilion

depth values, as a percentage of the average

diameter

8. Pavilion angle variation: the difference between

the maximum and minimum pavilion angle

values, in degrees

9. Girdle thickness variation: the difference

between the maximum and minimum girdle

thickness values, as a percentage of the average

diameter, measured at the bezel-main junctions

(see also features A1–A3 in figure 3)

10. Table size variation: the difference between the

maximum and minimum table size values, as a

percentage 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 symmetry

features combine to produce a general face-up visual

impression, so the symmetry grade is established by

looking at the face-up diamond. Depending on

where they occur, and how they combine, different

symmetry features can visually amplify or compen-

sate for one another, as discussed in box B. This

interaction plays a large role in determining the

overall symmetry grade for round brilliants with

lower symmetry. But for those with high symmetry,

the magnitude of a single feature may dominate the

evaluation.

Facet-related symmetry features also play a role in

determining 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, and

prominent naturals or extra facets are readily

observed, but they may occur independently of the 10

measurable symmetry parameters listed above.

Misshapen or uneven facets usually relate to a combi-

nation of the 10 parameters, but the relationships can

be complex.

RECOMMENDED SYMMETRY BOUNDARIES

The 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 the

diamonds. This comparison was repeated four times

over a period of 10 years, each time on newly

acquired data sets from several thousand diamonds.

Each analysis examined several sets of limits for the

10 parameters to identify robust matches with visual

symmetry grading.

Table 1 presents the ranges of allowed values for

individual symmetry features, measured in percent-

age or degrees, that GIA uses to support and con-

strain visual symmetry grading. The limits dividing

Fair from Poor symmetry are not presented here

because of the small number of round brilliants with

such low symmetry. Measured values should be

rounded to the indicated precision, if necessary,

before calculating the differences. If the value for any

one parameter falls into a range associated with a

lower grade, the overall symmetry grade will be low-

292 SYMMETRY GRADING FOR ROUND BRILLIANTS GEMS & GEMOLOGY WINTER 2011

he red dashed lines in the drawings in figure B-1

show 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 measured

values 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 and

table are off center in the same direction (case B), the

two symmetry features compensate for each other

visually. But when the table and culet are shifted off

center in different directions (case C), the negative

visual impression is amplified considerably.

When uneven crown height and girdle thick-

ness are added to the off-center table and culet, the

visual difference between various combinations of

these features becomes even more pronounced. In

figure B-2 (top), the table and culet are off center in

the same direction, and the girdle and crown height

are uneven along this same A-B axis. Arranged in

this way, these features tend to compensate each

other visually, particularly in the face-up view. In

contrast, figure B-2 (bottom) shows a table and

culet that are off center in opposite directions, and

the girdle thickness and crown height are uneven

in a different direction (along the G-H axis). This

combination amplifies the visual impression of

asymmetry.

BOX B: COMBINATIONS OF SYMMETRY FEATURES

Culet and table centered Culet and table off center

4% in same direction

Culet and table off-center

4% in opposite direction

Figure B-1. A culet and

table that are off center in

different directions produce

a more asymmetrical

appearance (case C) than

when they are off center in

the same direction (case B).

Note that the degree of

asymmetry is extreme,

down to the Fair range.

ered accordingly. Combinations of symmetry fea-

tures, as well as facet-related features that are not

measured, will still be evaluated visually, which

may also contribute to a lower symmetry grade.

For example, if nine of the parameters are within

the Excellent range but the table is off-center by

0.7%, the best possible symmetry grade is Very

Good. If all 10 parameters are within the Excellent

range, the expected symmetry grade would be

Excellent. But consider a round brilliant that is out

of round by 0.7%, with crown angle variation of 1.1°

and girdle thickness variation of 1.1%. Even though

all 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 1.9–3.6

Table off-center (%) 0–0.6 0.7–1.2 1.3–2.4

Culet off-center (%) 0–0.6 0.7–1.2 1.3–2.4

Table/culet alignment (%) 0–0.9 1.0–1.8 1.9–3.6

Crown height variation (%) 0–1.2 1.3–2.4 2.5–4.8

Crown angle variation (°) 0–1.2 1.3–2.4 2.5–4.8

Pavilion depth variation (%) 0–1.2 1.3–2.4 2.5–4.8

Pavilion angle variation (°) 0–0.9 1.0–1.8 1.9–3.6

Girdle thickness variation (%) 0–1.2 1.3–2.4 2.5–4.8

Table size variation (%) 0–1.2 1.3–2.4 2.5–4.8

SYMMETRY GRADING FOR ROUND BRILLIANTS GEMS & GEMOLOGY WINTER 2011 293

Figure B-2. These two round brilliants have multiple measurable symmetry faults that limit them to

no better than a Good symmetry grade. Although both stones have equal culet off-center values, the

appearance of overall symmetry is different because of the relative placement of the various symmetry

faults. The green crosshair indicates the center of the outline, blue is the center of the table, and red

denotes 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 of

asymmetry is amplified (bottom). In either combination, these displacements are considerably more

subtle than those shown in figure B-1.

Excellent, the combination of these three symmetry

features (and any others found on the diamond) may

result 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 lower

the symmetry grade, we recommend a “safety mar-

gin” for the trade to use in estimating the symmetry

grade. Accordingly, the values shown in table 2 are

20% lower than those in table 1. When the values

for all 10 parameters fall within these narrower rec-

ommended borders, there is a strong likelihood that

the 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 1.5–2.8

Table off-center (%) 0–0.5 0.6–1.0 1.1–1.9

Culet off-center (%) 0–0.5 0.6–1.0 1.1–1.9

Table/culet alignment (%) 0–0.7 0.8–1.4 1.5–2.8

Crown height variation (%) 0–1.0 1.1–2.0 2.1–3.9

Crown angle variation (°) 0–1.0 1.1–2.0 2.1–3.9

Pavilion depth variation (%) 0–1.0 1.1–2.0 2.1–3.9

Pavilion angle variation (°) 0–0.7 0.8–1.4 1.5–2.8

Girdle thickness variation (%) 0–1.0 1.1–2.0 2.1–3.9

Table size variation (%) 0–1.0 1.1–2.0 2.1–3.9

measurement. Within these recommended limits, it

is unlikely that a combination of measurable sym-

metry features would lead to a lower symmetry

grade. Note that the second example in the previous

paragraph exceeds two of these recommended limits.

The boundary values presented for these 10 sym-

metry features are most useful along the

Excellent–Very Good symmetry border, where a sin-

gle feature often dominates the final grade determi-

nation. These individual parameter limits are also

relevant for the border between Very Good and

Good. When symmetry problems become severe,

though, it is more likely that multiple symmetry

features will limit the grade, because the interac-

tions among symmetry factors become more pro-

nounced (again, see box B). Because combinations of

minor symmetry features can create a significant

visual impact, the limits in the tables must be

viewed only as a guide.

DISCUSSION

During the analysis of laboratory grading results, we

observed some variation in how strictly symmetry

was evaluated by our graders, particularly for mea-

294

SYMMETRY GRADING FOR ROUND BRILLIANTS GEMS & GEMOLOGY WINTER 2011

Figure 4. These three

round brilliants each dis-

play a combination of

symmetry faults. (A) The

table of this 1.00 ct dia-

mond (Fair symmetry) is

not an octagon (6.1%

table size variation, as

shown by the blue and

yellow lines) and the table

is off-center by 2.5%. The

asymmetry of the table is

associated with crown

angle variations and

uneven bezels (marked

red). (B) The culet of this

0.83 ct diamond (Fair

symmetry) is off-center by

2.9% (red dot). The table

is also off-center in an

opposing direction (green

dot), yielding a value for

table/culet alignment of

3.4%. These symmetry

faults are associated with

uneven bezels (marked

red) and pavilion mains.

Unlike the diamond in A,

the nearly equal quad-

rants defined by the yel-

low lines show that the

table is octagonal. (C) In

this 0.69 ct diamond (also

shown in figures 1 and 3;

Good symmetry), the gir-

dle is wavy and not paral-

lel to the table. Photos by

GIA (A and B) and Robert

Weldon (C).

SYMMETRY GRADING FOR ROUND BRILLIANTS GEMS & GEMOLOGY WINTER 2011 295

REFERENCES

Blodgett 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 Grading

System. 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., Geurts

R.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 for

the GIA Diamond Cut Grading System. Rapaport Diamond

Report, Vol. 28, No. 30, pp. 34–39, www.gia.edu/diamondcut/

pdf/0805_pg34_39.pdf.

Uncertainty of measurement results (1998) The NIST Reference

on Constants and Uncertainty. http://physics.nist.gov/cuu/

Uncertainty/international1.html [date accessed: June 14, 2011].

ABOUT THE AUTHORS

Mr. Geurts is a manager of research and development at GIA

in Antwerp. Dr. Reinitz is a project manager at GIA in New

York. Dr. Blodgett is a research scientist, and Mr. Gilbertson a

research associate, at GIA in Carlsbad.

sured features near the border between Excellent and

Very Good. A common set of fixed numerical limits

for these parameters can only improve the consisten-

cy of symmetry grading for such stones. Diamonds

with at least one parameter beyond the limits shown

in table 1 will receive the lower symmetry grade.

Symmetry features not captured by these 10 param-

eters will continue to be evaluated visually. If these

additional facet-related features are sufficiently

prominent—an extra facet polished at the corner of

the table, for instance—they will reduce the sym-

metry grade even if all measured parameters fall

within the narrower limits in table 2. Visual sym-

metry observations cannot raise a symmetry grade,

but they can reveal instances when a cleaner, more

correct measurement of the diamond is needed.

Measured values can be of great help for dia-

monds with multiple symmetry faults, such as the

three shown in figure 4. In such cases, some of the

symmetry features are more easily noticed visual-

ly, while others are captured more accurately by

measurement. In figure 4A, the asymmetry of the

table leads to variation in crown angles and

uneven bezel facets. In other cases, similar faults

with the table might be associated with a wavy

girdle that takes up the uneven aspects of the crown

and allows little variation in the crown angles. Under

both sets of circumstances, the uneven bezels are a

prominent feature that does not describe the underly-

ing symmetry faults as clearly as the measured values

for crown angle variation, crown height variation, and

girdle thickness variation.

In figure 4B, the off-center culet and table lead

to uneven bezels and pavilion mains. The displace-

ment between the table center and the culet

emphasizes the visual impact of the off-center

culet (again, see box B), but the measured values—

that is, Good for table-culet alignment, but Fair for

table off-center—provide a context for evaluating

the severity of the combination. In figure 4C, the

most prominent symmetry fault is displayed for

the diamond shown in figure 3. The table and gir-

dle are not parallel, a fault that is more severe than

the uneven girdle thickness or the facet-related

symmetry features.

CONCLUSION

Measurement is a process full of inherent uncertain-

ties, but GIA’s efforts to achieve smaller uncertain-

ties have been successful. Starting in early 2012, the

measurable values presented in table 1 will be used

to attain greater consistency than is possible

through 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 for

manufacturers, to help ensure that the final symme-

try grade will not be undermined by combination

effects or measuring tolerances.