After the close of the GIA Symposium in San Diego all four authors were able to experiment with (but not photograph in) the GIA Diamond Dock® as we all attended a GIA Cut Grading Course.

We noted different instructors assessed 'brightness' using differing lighting/observer angles with the light box (and other light sources, such as the small fluorescent lights on microscopes. When asked about the correct lighting/observer angle, the instructors could offer no consistent or quantifyable recommendation.

In the original Diamond Dock® Users Manual, a recommendation is made that the observer views diamonds at an angle of 45° to the vertical plane, which we believe was the same angle observers used for the survey testing of 70,000+ observations that formed the basis of GIA's new Cut Grading System. Anecdotal information from volunteers who participated in GIA's observation testing seems to confirm that observational tests were performed in a seated position that would have subtended an angle of 40° to 45° between the stones and the centre of the lights (depending on observer height).

When the incident light from the fluorescent tubes comes from this angle, slightly shallower diamonds have less brightness in Diamond Dock®, whereas Tolkowsky proportioned and deeper diamonds appear brighter. We were able to demonstrate how important this viewing position is to two GIA instructors, who by their own choice, viewed diamonds from a much higher view point than 45°—one where the incident light would have been subtending an angle of approximately 20° to 30° relative to the vertical and the light source. We had with us two diamonds of similar colour, clarity and diameter. One diamond of 1.19ct, with proportions very near that of Tolkowsky's, had been graded Excellent by GIA Gem Trade Lab. The other shallow 1.16ct stone graded Very Good (a border line Good when using Facetware™). From the higher viewing angle, both instructors preferred the shallow stone for brightness (and were split one each for 'fire' using the instrument's LED lighting only.

Figure 5 shows ETAS data based on images produced from three dimensional models of these two stones made with DiamCalc® along with the actual steep deep diamond used in our original article. This demonstration can be used to explain why the shallower stone appears brighter when the incident light comes from a higher angle; for the shallower diamond draws and returns more light from directly above, while as round diamonds get deeper they draw more light from lower angles closer to the horizon. Readers are invited to use similar modeling software at this website: www.cutstudy.com/cut/english/grading1/sphere1.htm

Shallow 1.16ct GIA VG	'Ideal' 1.19ct GIA Ex	Deep GIA 1.28ct Ex
Potential to see the Diamond Dock lights in a diamond from a 45° viewing angle
Potential to see the Diamond Dock lights in a diamond from a 23° viewing angle
Fig. 5. Effective Total Angular Size (ETAS) images modeled in DiamCalc based on 3D scans from three actual diamonds. The flares of light show the potential and direction that light in the environment around each diamond could come from to cause a sparkle in the face up position. The lines mark 10° increments on a virtual dome, with the zenith in the centre facing the observer. The diamond is shown as a small blue dot in the centre. The 6 arcs in the upper rows represent the position of the diffuser over two fluorescent tubes in Diamond Dock.

The steeper lighting-viewing angle makes most round diamonds look brighter, which probably explains why Diamond Dock® users seem to naturally prefer to view diamonds from higher angles than the original designers intended. We believe GIA would have arrived at shallower Excellent proportion parameter sets had the observers in their observational tests been directed to use a higher viewing angle, but then these proportions may have been too shallow.. However, as we have already shown, the light background and very close proximity and brightness lights in the CVE lighting box its narrow source of angular size of lighting and lack of consistency in viewing angles make this method of cut grading useless. Basing a cut grading system on observation and survey results from this method is, we believe, flawed. In particular this lighting environment would have even less validity for brightness grading of fancy shaped diamonds, many of which tend to gather very little light from the illuminated area in the CVE.