The proper assessment of cut in diamonds has long been an elusive, but intriguing, goal. As the authors point out in the lead article, most diamond grading systems in use today establish parameters for cut grades in round brilliants based on a variation of proportions devised by Marcel Tolkowsky in 1919. For nearly 80 years now, there have been few, if any, rigorous attempts to shed more light on the subject.

Recognizing this fact, several years ago the Gemological Institute of America made a long-term commitment to establish a comprehensive understanding of the effects of cut and proportions on diamond appearance. Our goal was, and is, to develop modern criteria for cut assessment using today's sophisticated technology, and to integrate this knowledge into our diamond education and training courses. When the study has been successfully completed, this knowledge may even be applied to our laboratory reporting and instrument development.

This much-anticipated first article on GIA's three-dimensional, ray-tracing computer modeling addresses the most important appearance concept, brilliance, based on what the authors call 'weighted light return.' As the authors state, a brilliance measurement is one of several important pieces of the 'cut' puzzle; dispersion, scintillation, and perhaps symmetry and color, are others. Since a polished diamond should display a pleasing combination of brilliance, fire, and scintillation, the elusive 'best' overall appearance might not be found among just the brightest round brilliant cuts. Thus, we caution you to read this article fully and carefully, and to refrain from drawing unequivocal conclusions from this initial work.

So what do we know at this point? Certainly, we know that cut is the most complex of the 4 C's, even when isolated to one cutting style: the round brilliant. We know that the success of cutting for weighted light return has more to do with the interrelationship between three critical measurements--table size, crown angle and pavilion angle--than on a selection of isolated proportion measurements. We know that one cannot, and must not, assess the cut of a diamond by examining any one of these proportion parameters alone.

We also know that there are many combinations of proportions that yield equally attractive round-brilliant-cut diamonds. In fact, we know that diamonds can be cut in a fairly wide range of proportions to yield the same high light return, which can lead to better utilization of the rough and a better fit with the myriad tastes that exist in the global marketplace.

Finally, we know from our extensive historical research on cut that there have been numerous claims to a single set of 'Ideal' proportions in round-brilliant-cut diamonds. These have ranged from Wade's American Ideal in 1916--with a 45.3% table, a 35° crown angle, and a 41° pavilion angle--to Watermeyer's Modern Ideal in 1991 (with a 61% table, a 34° crown, and a 41° pavilion). The derivation and use of the term 'Ideal' is thus confusing at best, but not unlike that of 'blue-white' and 'perfect' decades ago. 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.

William E. Boyajian
President, Gemological Institute of America