GIA Releases Brilliant Diamond Cut Study

November 20, 1998

GIA Releases Brilliant Diamond Cut Study

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

"The use of the term 'Ideal' is thus confusing," GIA President William E. Boyajian says in the 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 modern times."

To come to its conclusions, GIA created a computer-generated three-dimensional model of a round brilliant colorless flawless 58-facet diamond with perfect symmetry and polish. It entered eight cut factors and several physical factors that affect the way light interacts with diamond. These calculations resulted in more than 20,000 proportion combinations which were measured for brilliance through a numerical evaluation GIA calls weighted light return (WLR). The WLR numbers were reached by tracing and weighting rays of light as they followed their paths through the diamond.

Some well-known cut combinations yielded surprising WLR brilliance values. Marcel Tolkowsky's historic Ideal proportions fared well, producing a "moderately high" WLR value in GIA's brilliance category, while American Ideal proportions produced a WLR in the "moderately low" category. GIA also compared the proportion combinations of more than 67,000 diamonds the Gem Trade Laboratory previously graded with the proportion combinations created by the virtual diamonds in the mathematical model. In this part of the study, natural and virtual diamonds with comparable proportion combinations demonstrated comparable brilliance.

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

For a copy of "Modeling the Appearance of the Round Brilliant Cut Diamond: An Analysis of Brilliance" appearing in Gems & Gemology, call GIA at (800) 421-7250, ext. 7138. Single copies cost $16.95.

- by Robert Weldon, G.G.