Researchers at the Gemological Institute of America (GIA) have published findings from the second of their multi-part study on diamond cut - this one concerning the effect of proportions on "fire" in round brilliant diamonds - and the results appear in the Fall 2001 issue of the Institute’s peer-reviewed journal, Gems & Gemology, mailed to subscribers Nov. 1. The article is titled "Modeling the Appearance of the Round Brilliant Cut Diamond: An Analysis of Fire, and More About Brilliance."

In this segment of their comprehensive study on diamond cut, the researchers used a computer-modeled "virtual" diamond, and to analyze fire they developed a metric they call dispersed colored light return (DCLR). A prime conclusion reached by the research team is that there is no single "best" set of proportions for a round brilliant that maximizes fire. Instead, as they also found in their earlier study of brilliance (evaluated as weighted light return, or WLR), published in the Fall 1998 issue of G&G, there are many different sets of proportions that can achieve this goal.

In this study, GIA researchers computed the DCLR value for more than 26,000 combinations of round brilliant proportions - including crown and pavilion angles, table size, lengths of the star and lower-girdle facets, culet size, and girdle thickness. They checked predictions from these metrics against images collected from 28 actual diamonds acquired or manufactured for the study and another 400 that had been submitted to the GIA Gem Trade Laboratory for grading reports. The researchers found many sets of proportions that produce average or above-average values for both fire and brilliance.

The researchers also found that the lengths of star- and lower-girdle facets may play an important role in improving both fire and brilliance, something ignored in most previous studies on cut and in current commercial cut grading systems. For example, according to the G&G article, lengthening the lower-girdle facets from Marcel Tolkowsky’s value of 50% to a "modern" value of 75% to 85% caused a considerable increase in DCLR. Similarly, varying the length of the star facets also affected DCLR, although the difference showed no obvious trend and depended on other proportion factors. Tolkowsky developed parameters for the modern round brilliant cut in a monograph published in 1919.

The GIA studies on brilliance and fire create the basis for a better understanding of how cut proportions work together in a round brilliant diamond. This information could lead to a greater degree of freedom for diamond manufacturers, providing improved yields from rough while achieving above-average fire and brilliance. For example, cutters may be able to adjust star, upper-girdle, and lower-girdle facets in the final stages of the manufacturing process to improve a round brilliant’s appearance. However, small variations in some proportions can have a large effect on appearance, so greater precision in manufacturing may be necessary.

GIA President William E. Boyajian has said the Institute plans to add more information regarding cut to Diamond Grading Reports issued by the GIA Gem Trade Laboratory in 2002.

The research team consisted of Dr. Ilene Reinitz, Dr. Mary L. Johnson, T. Scott Hemphill, Al M. Gilbertson, Ron H. Geurts, Barak Green, and Dr. James E. Shigley. The researchers are also providing an ongoing series of short articles, GIA on Diamond Cut, that can be viewed on the Research page of GIA’s Web site, www.gia.edu <http://www.gia.edu>.

To purchase a copy of the current Fall 2001 issue of Gems & Gemology, with the full study on fire, go to www.gia.edu/gandg <http://www.gia.edu/gandg/>; or contact Subscriptions Manager Debbie Ortiz at 800-421-7250 ext. 7142 (toll-free) or, outside the U.S. and Canada, at 760-603-4000, ext. 7142.