We would like to clarify the comments made concerning the diamond in figure 3 of our paper, with a 57% table size and comparatively steep crown (35.3° ) and deep pavilion (41.4° ). This diamond was given a grade of 'Excellent' cut by GIA (you can check its position on the chart in figure 2), but it did display very unattractive leakage just inside the table when viewed over skin or on a dark background. This leakage was not apparent when the stone was resting on the light coloured (18% gray) tray used in GIA's observation-based survey.
The GIA team had noted that diamonds appear different when observed (and also photographed) on backgrounds of differing shade. But, after personal communication with a key member of the GIA team, at the GIA Symposium August 2006, we do not believe they understand fully why.
We will show that the way our eyes adapt to a brightly lit and light colored background reduces our ability to make fine distinctions between diamonds. GIA's Common Viewing Environment (CVE) that was used in their survey environment had a background and stone trays of a shade of 18% gray, with twin diffused fluorescent tubes mounted less than 33 cm (13 inches) above the diamonds. This causes an observers eyes to 'overexpose', creating a bad environment for discerning differences between diamonds of differing brightness. The authors of this response spent a little time with the commercially available Diamond Dock® version of the CVE at the GIA Symposium in August 2006, This confirmed that the viewing environment is quite similar to a diamond dealer's twin fluorescent desk lamp and viewing position. Yet, when asked to evaluate the brightness of a diamond, buyers and dealers usually do not use the desk lamp for this purpose. Instead, they typically examine diamonds under the edge of the desk, with their body turned away, using ceiling lighting near a window or even in a hallway under spot lighting.
At the Symposium, the Cut Group authors conducted a brief experiment with GIA's Diamond Dock® CVE and took the two photographs shown in figure 2. We apologise for the quality of these images; but we were stopped by a GIA Director who protested that we were conducting experiments "to use against GIA". Note that our eyes, and a camera with auto exposure, adapt to the background brightness in a similar manner. What we 'saw' was indeed similar to the photographs shown in figure 2.
With the twin fluorescent lamps turned on (Fig. 2, LHS, when looking at these sets of coloured cubic zirconia , which were used for the poster presentation of Sergey Sivovolenko and Yuri Shelementiev, the princess and oval cut stones appeared brighter than the ideal cut rounds. With the lights turned off and only with ambient room illumination (Fig. 2, RHS), the relative brightness of these various cuts gave the opposite results, the results one would usually expect to see, for the round stones appeared brighter. It is indeed unfortunate that as the stones were adhered to black cards, as this somewhat lessened the overall effect of the bright background illumination that we were seeking to demonstrate.
Fig. 2. These coloured cubic zirconia of different cuts were photographed inside GIA's common viewing environment Diamond Dock®. The photograph on the left was taken with the fluorescent grading lights turned on. For the photograph on the right the lights were turned off and only the ambient room light was illuminating the stones. Note that with the lights on the oval and princess cuts appear brighter than the well cut rounds. The results are the complete opposite when the brightness grading lamps are turned off, with the rounds appearing brighter. The reason for these differences is that the camera's auto exposure adapts to the background in a similar way as our eyes.