A review of the “foundation” of GIA’s new cut grade system

The first and last stones appear to be fair mid grade stones. The middle stone lacks observer’s presence contrast; I believe it would look worse than it appears in the photo.

RD22

This stone could possibly have been improved by the manufacturer; by lengthening the lower girdle facets and shortening the star facets (while holding the other proportions constant) would dramatically improve scintillation and would reduce the darkness caused by the short lower girdle facets which are the cause of the thick dark star in the table.

A second and better possibility may also have existed. Diamonds cut to these proportions are frequently the only stone polished produced from the rough. They are called ‘makeables’ because they are made without sawing from often rounded ‘dodecahedra’ rough. Of course we can not know if it this would have been possible – but often very steep crown angle diamonds can be made from such material. If it was possible in this instance, then making the crown angle 2° steeper would have resulted in a stone that scored HCA 1.2 FIC which is an abbreviation for Firey Ideal Cut. This stone would have a 4% better yield.

DiamCalc model - GIA data Model with new minor facets New minor facets & +2° crown
Light Return Mono 0.84 0.89 0.91
Light Return Stereo 0.83 0.92 0.93
Contrast 0.99 1.14 1.14
Star facet length 55% 45% 45%
Lower G’s 75% 90% 90%

In the article, GIA only mentions one stone as being a slight fisheye (RD37 Cat 4), but infact this stone, even though it has a small table size, would show a slight fish-eye (as would RD19 Cat 4).

RD11

If this survey had involved consumers actually purchasing diamonds, spread and apparent size would have been a very important factor in the grade setting of this and some other stones. This mid grade stone is a clear example of the difference between the institutional and the real worlds. The dark upper girdle facets in this stone (as mentioned in the article) are a result of leakage that is evident in its DiamCalc modelled ideal-scope image. Upper girdle facet leakage results in reduced ‘apparent size’ or ‘spread’ because our eyes loose the ability to distinguish the edge of the diamond; i.e. it looks smaller than its actual diameter. RD11 weighs 9% more than we would expect a nice diamond with this diameter to weigh, but a diamond with the same ‘apparent’ face up spread would about three quarters of the weight. This diamond would cost half as much again as a better cut diamond with a similar apparent size.

In my opinion ranking a diamond like this as a mid grade stone could result in retailers and consumers accusing GIA of pandering to large manufacturers and dealers, some donate generously to the GIA’s League of Honour Fund.

This diamond raises the possibility of a weakness in the lighting arrangement that GIA used. The DiamCalc ray trace shows that two facet sets are illuminated by light coming predominantly from the same light source. If we imagine placing this stone on a Brilliancescope®, with its annular or circular light source that moves up and down perpendicular to the diamonds table, we would expect the diamond to remain dull unless one of the 5 Brilliancescope® reading positions happens to fall in that region. In the DiamCalc ASET™ model, employing a variation on an Ideal-scope / Gilbertsonscope * approach, we see that the bright portion of the stone in the GIA photo are predominately pink.

The ray trace diagram shows that the center of the table, and most of the crown main facets, are both illuminated from the same direction; this shows as the red on the AGS ASET DiamCalc lighting model. The stone would appear dead if light from those directions was unavailable. (The ASET blue illumination would be largely obscured by an observers head. The green colour indicates light that has come from close to the horizon, the least likely source of bright illumination.)

RD06

GIA’s computer model predicted lower crown angle stones would have better brilliance; naturally they needed to test this with visual observations; stones were polished especially for this purpose. The computer scientists must have been disappointed by the results. On page 225 two reasons are given for mediocre performance. Firstly they mention low values for crown height and angle and secondly they refer to the stones lack of contrast and scintillation. I believe they made the pavilion angle too shallow. (It would be good if GIA stopped referring to “Crown Height”. A diamonds crown height is determined by two factors; crown angle and table size. Crown height per se has little or no determining impact on a diamonds appearance.)

A low crown height needs to be combined with an appropriate pavilion angle. GIA predictions of high light return for RD06 (WLR 3.01) in their 1998 G&G light return study failed to account for an observers head, a fact well known and discussed adnauseum. Also well known is the head obscuration of shallow crown stones has a more deleterious impact than with other proportion sets. Because they made the pavilion too shallow f particular crown height, the stone shows what all shallow stones show – too much darkness. The darkness could have been reduced with adjustments to minor facets, but a better approach would have been to cut a deeper pavilion as in the example below (A pavilion angle in between would probably have yielded a more balanced stone). It appears that GIA have failed to heed the feedback from critics and have not made appropriate adjustments to their brilliance software.

The stone on the left is a DaimCalc model of the GIA stone. On the right is this stone with similar proportions, but 1° deeper pavilion; it scores an amazing 1.11 and 1.08 for DiamCalc light return, but rates only 0.75 for contrast (questioning the Brilliancescope® results for scintillation).

GIA have made no mention that shallow crown angle is a durability issue. Perhaps GIA will follow AGS’s lead and downgrade shallow crown angles for durability in the finalized system?

* Al Gilbertson is now one of the GIA Cut Study team