Here is the Cut Group letter to the Editor in reply. (We have named ourselves the Cut Group for several years. The GIA had previously referred to themselves as the Cut Study Team).

Dear Editor,

Thank you for giving us the opportunity of responding to the letter of critique sent to the Editor from the GIA Diamond Cut Team concerning our paper in the April-June 2006 issue of The Australian Gemmologist.. This response from our Cut Group* represents a rare opportunity to engage in open dialogue with the GIA Diamond Cut Grading research team, and we also whould like thank them for this opportunity.

With regard to differences between the GIA's and American Gem Society's (AGS) grading systems; we have choosen only to highlight examples of extreme grade differences between the two systems to avoid the very argument outlined in the GIA Cut Grading research team's letter "that there is much overlap in the cut grades from the two systems". We have taken the GIA team up on this challenge by comparing the overlap between the AGS's 11 grades and GIA's 5 grades for the common 'Excellent' table size of 57%. Comparing that overlap is difficult because GIA arranged its pavilion axis opposite to the generally accepted chart format that AGS (and other researchers) have traditionally used. However, Peter Yantzer, the Director of the AGS Lab, kindly provided us with the AGS 'candidate' charts in the GIA's format (as displayed at www.diamondcut.gia.edu/charts/ ). A comparison of the 57% table size charts showed that GIA 'Excellent' proportions (outlined in black on figure 2) had only 26 good AGS0 and AGS1 matches, and 31 bad matches. Included in these bad matches were several very bad matches—specifically 7 AGS 5s and 4 AGS 6s that would be graded Excellent by GIA (see table 1 and figure 1 for details). We consider this to be a poor match and, in addition, confusing for both consumers and the industry. The situation is worse for table sizes below 52%, where GIA has no 'Excellent' proportion sets. For example, at the 51% table size AGS has 17 AGS0 and 22 AGS1 'candidate' proportion sets that GIA would grade as Very Good, Good or Fair.

Table 1 The good and bad matches between different grades of GIA and AGS 'candidate' grades.
(The authors deliberately excluded AGS 2 because in a 5 v 11 grade comparison no significant conclusions can be drawn.)

Good matches		Bad matches
GIA Ex & AGS0	13	GIA VG & AGS1	12
		GIA Ex & AGS 3	3
GIA EX & AGS1	13	GIA Ex & AGS 4	5
		GIA Ex & AGS 5	7
		GIA Ex & AGS 6	4
26 good matches		31 bad & very bad matches

Fig. 1. The AGS 'candidate' proportions by pavilion and crown angles for table size 57%. The black overlain shape contains the GIA 'Excellent' proportions used to produce the data in table 1.