We did not claim that green is never seen in polished diamonds. What we said was "rarely". We too hope that all readers will do some simple observations in normal lighting too to discover for themselves how rare green flashes are. It would appear that the GIA team has confused what the eye can see when a sparkle of light from a diamond shines onto a surface, such as a dome, with what the eye would see if the dome was removed and an observer looked from this position with their naked eye. Most flashes we see with our eyes appear white because the spectral colours would have merged together. To help explain why green is rare we have paraphrased a good explanation of 'fire' by Anton Vasiliev that was published 12 years ago in a work still available in full at www.gemology.ru/cut/english/rainbow/5.htm )

Consider the observer, which GIA did not in their published studies. For this observer with a pupil diameter of 3 mm, a viewing distance of 25 cm creates an angular size of 0.7° to allow for the possibility of light entering the eye. For an observer to see a 'pure' spectral colour, a gem must have a high 'fan' of angular dispersion. But because green is in the central region of the spectrum, there is a high chance that green will be mixed with other colours, and appear whiter (see figure 3, left side for an example). For this reason, the greater the distance between the gem and the observer, the more pure the 'fire' one can see.

Now consider the type of illumination , which GIA also did not do in their published 'fire' studies. To create a full spectrum, a prism should be illuminated with a light that has a very small angular size—such as that from the sun, a fiber optic source shining through a pin hole, or a small lamp at a very great distance. In contrast, when a light is larger and closer, as in most indoor situations, the light beams fan out and cause the spectral colors to mix, degrading the purity of the observed color. Hence, good green fire requires illumination of the gem from a light source of small angular size. The source's angular size affects the color purity in a similar way as that of the the size of the pupil in the observer's eye..

Fig. 3 These images show OctoNus Effective Total Angular Size (ETAS) DiamCalc models for various viewing conditions. (Click on the pictures to open larger images)

The image on the left is based on the information provided in the GIA letter to the Editor and shows the full spectrum of flashes that could be seen on the surface of a dome. On the lower right of the left side image a small circle shows that part of the flash that could enter a 4 mm pupil if the dome was removed and an observer looked from this position with their naked eye. Under these circumstances the flash would appear white because the spectral colours would merge together. If the observer moved a little up and to the left they would see a reddish flash, and if they moved down slightly to the right, the flash would appear blue as in the example shown here www.gemology.ru/cut/english/grading1/5.htm. The image on the right shows the probability of the type of flash an observer might see from 25 cm if the source of light was 10 mm in diameter and was positioned 2 metres away from the diamond. In reality (with the dome removed) the probability of seeing green is very small.

As we have shown in figure 3, the illumination and observer are just as important as the diamond itself. We believe the GIA used inadequate illumination models for their virtual studies, and to the best of our knowledge, they have not studied or published any work concerning observer's eyes or their vision systems.

The diamond on the left of figure 4 has been photographed in office lighting and shows predominantly blue coloured flashes, with some red and yellow. As we have explained green flashes are harder to see if the light source is large in size, and relatively close to the diamond. The diamond on the right shows some green; for it was photographed in partly shaded direct sunlight, effectively parallel rays of light. In addition, both pupil and camera apertures' are small because we are outdoors in very bright daylight. Our narrow pupil aperture allows us to see some green flashes without the merging of yellow and green or blue and green. An additional explanation can be found here at www.gemology.ru/cut/english/grading1/4.htm

Fig. 4. The 15 ct diamond on the left has been photographed in office lighting where green will rarely be seen because the lights in the office have a large angular size. The 9 ct diamond on the right shows some green; for it was photographed under a broad leafed tree in partly shaded direct sunlight, effectively parallel rays of light. In very bright daylight both pupil and camera apertures' are small which allows us to see more green flashes without the merging of yellow and green or blue and green. Both photographs were taken with the same Canon Ixus camera, set to auto exposure mode, with the diamonds positioned on author GH's fingers.