The GIA authors' contention that they have struck a balance between shallow and deep diamonds does not concur with the widely accepted Tolkowsky standard. Holding the pavilion angle at 40.8°, the GIA 'Excellent' grade's average crown angles for table sizes between 52% to 62% are around 1.5° steeper than Tolkowsky's 34.5° crown angle. Alternatively, holding the crown angle constant, GIA's Excellent proportions have an average pavilion angle that is 0.3° deeper than that recommended by Tolkowsky. Clearly the GIA cut grading system favours deeper diamonds than the generally accepted optimum proportions.
The GIA team wrote in their letter: "we state that during our research we found that a diamond's relative sparkle was accounted for in our brightness and fire metrics". Unfortunately, to date, GIA have never reported any metric that measures a "diamond's relative sparkle" or scintillation. As their brightness and fire metrics had only a 58% match to the observation results, and since the GIA have never developed a sparkle or scintillation metric, how can they claim a 92% match between the metrics and observation results? This seems to be a circular argument? It would appear that GIA's first 10 years of virtual diamond modeling study have been of little or no value. This is unfortunate. For GIA does not, as they agree in their letter, have the tools to develop cut grading systems for fancy shaped diamonds.
Our reference to 38 million proportion combinations as being excessive was related to fancy shaped diamonds. Such a data base would be thousands of times larger and more complex than that for the round stone. Also it probably would be unworkable for rough diamond planning.
The round data base is relatively simple, and the vast majority of the 38 million data points are of no use to manufacturers who are only interested in the boundaries of Excellent and Very Good cuts. After GIA released its cut grading charts in February 2006, our information is that sales of the expensive Facetware® in Sarin and Ogi rough diamond planners have all but ceased.