GIA's Brilliant Diamond Cut Study

January 1999


GIA's Brilliant Diamond Cut Study

There's more than one way to make a beautiful diamond

The first part of the Gemological Institute of America's long-awaited analysis of diamond cut confirms the long-held belief there is no one Ideal cut – at least where brilliance is concerned.

The study finally proves scientifically what good jewelers and diamond cutters have always known: a diamond doesn't have to be cut to a few rigid parameters to achieve superior brilliance.

"The use of the term 'Ideal' is thus confusing," GIA President William E. Boyajian says in his introduction to the study, published in the Fall 1998 issue of Gems & Gemology. "Although it is not GIA's role to discredit the concept of an 'Ideal' cut, on the basis of our research to date, we cannot recommend its use in modern times."

GIA studied brilliance first because it's thought to be the most important appearance factor. The study will continue with analyses of two other key diamond appearance factors, scintillation and fire, as well as the effect factors such as color and symmetry have on diamond appearance. Only then can GIA evaluate the full effect of cut on a diamond's appearance.

Rolling Out the New Model
To produce data on how cut affects brilliance, GIA created a computer-generated three-dimensional model of a round brilliant, colorless, flawless, 58-facet diamond with perfect symmetry and polish. It then entered eight cut factors (crown angle, pavilion angle, table size, culet size, star length, lower girdle length, girdle thickness and girdle facets) and several physical factors that also affect the way light interacts with a diamond.

This three-dimensional virtual diamond is a vast improvement on the two-dimensional model developed in 1919 by legendary diamond cutter Marcel Tolkowsky (see Professional Jeweler, August 1998, p. 40). Tolkowsky's groundbreaking analysis first mathematically quantified cut and created a standard by which all round diamonds could be compared. Ultimately, this led to a single standard – with no deviations – that came to be known widely as the Ideal cut. Since then, deviations from Tolkowsky's model have proliferated.

Measuring Brilliance
Because of the growing number of claims that certain proportions create the best diamonds, there's been increasing demand for a cut grade. GIA has been reluctant to issue cut grades without an exhaustive scientific foundation for them.

To begin to create that foundation, GIA used its virtual diamond to create 20,122 proportion combinations, primarily by mixing the key cut factors (crown angle, pavilion angle and table size) while keeping the other five cut factors constant. (It also examined the other five cut factors separately.) For each of these combinations, GIA measured brilliance through a numerical evaluation it calls weighted light return (WLR).

The WLR numbers were reached by tracing and weighting rays of light as they followed their complicated paths through the diamond, interacting with the diamond and revealing its brilliance. GIA created a standard lighting situation that simulated the average daylight in which diamonds are viewed. It also factored in the way diamonds are often "rocked" when they are viewed and excluded glare, which could be mistaken for WLR.

The GIA researchers concluded from their findings that "the relationship between brilliance and the three primary proportion parameters [crown angle, pavilion angle and table size] is complex..." The way these factors interact with each other can affect brilliance in sometimes unexpected ways. Ultimately, a number of different cut combinations produced diamonds in a desirable range of brightness.

Surprisingly, some well-known cut combinations yielded lower- or higher-than-expected WLR, or brilliance, values. Tolkowsky's diamond proportions, for example, produced a WLR in GIA's "moderately high" brilliance category. American Ideal cut proportions yielded a WLR in the "moderately low" category for brilliance.

GIA also looked at some diamond proportion combinations used by various labs that bestow a cut grade. When the proportions of the American Gem Society Laboratory's "0" grade diamonds ("0" is that lab's best cut grade) were examined, for example, they generated WLR values that ranged from "typical" to "moderately high" in GIA's categories. But the range of proportions for AGS "5" graded diamonds ("5" is a lower grade in the AGS system) yielded some WLR values in the highest GIA brilliance category (see Professional Jeweler, September 1998, p. 30, for a detailed look at AGS cut grades).

GIA augmented its study by comparing the proportion combinations it created for virtual diamonds with 67,621 diamonds previously examined in GIA's Gem Trade Laboratory. The finding: virtual and natural diamonds with comparable proportion combinations demonstrated comparable brilliance.

The study's authors cautioned strongly against using WLR brilliance categories from the study as "brilliance grades" because much work remains to be done before GIA can make any definitive statements about cut's effect on overall diamond appearance.

This illustration (left) shows the track of one light ray within a round brilliant diamond, as calculated by GIA's mathematical model.


At top is a computer-generated profile of a diamond used in the GIA study. See how the model captures many aspects of an actual diamond, including three-dimensionality, transparency, facet arrangement, overall light return, pattern of light and dark reflections and fire. The other two illustrations show a virtual diamond (center) and real diamond (right) with comparable proportions. Illustrations & Photo by Vincent Cracco (right) are courtesy of GIA

– by Robert Weldon, G.G.

"Modeling the Appearance of The Round Brilliant Cut Diamond: An Analysis of Brilliance," appeared in Gems & Gemology, Fall 1998. Its authors were T. Scott Hemphill, Ilene M. Reinitz, Mary L. Johnson and James E. Shigley. To order a single copy of the issue ($16.95), contact Gems & Gemology, Carlsbad, CA; (800) 421-7250, ext. 7138.

Copyright © 1999 by Bond Communications.


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