A chess board is 50% black; it returns half as much light as a sheet of white paper. But a chess board looks brighter than white paper when they are both moving. The same is true for a diamond; if a stone returned light so that the crown was bright all over, it would not appear as bright to your eye s as one with good contrast. While light return is an important part of ‘brilliance’, our perception of brightness is enhanced by our eyes adaptation to contrasting backgrounds. All six patterns in Fig 1 are equally bright to a light meter, but those with good contrast are more brilliant to our eyes. Diamonds with good contrast, caused by alternating very bright and very dark facets, appear brighter because of the psycho-physiology of the way we see. Unfortunately light meters can not be programmed to work like the human mind and this is one reason why designing diamond cut grading systems is very difficult.
Figure 1. Each image has the same overall brightness or light return (according to a light meter)
Take for example two exactly equal diamonds of 5ct, and 0.30ct. They have different facet sizes and to our eyes they appear to have different brilliance and scintillation. The top right hand pattern in Fig 1 has squares that are too small to be distinguished by our eyes, like many of the facets on a smaller diamond, but some facets on the larger stone could be too large, like those on the top left.. When asked recently about this example, the GIA Cut Study team acknowledged that there would be a difference in appearance, but that their new cut grading system would give each stone the same grade. A Cut Group cut grading system will account for such differences.
A second example is the princess cut, which has 12 less facets than a round brilliant, but appears to have many more small ‘virtual facets’ than a round diamond. Large princess cuts are popular but smaller princess cuts are not so much in demand; one reason is the ‘virtual facets’ of small princess cuts are too small for the ‘physiological’ ability of our eye to separate them. Facets merge together, like the top right image, resulting in negative or poor contrast and thus less brilliance, sparkle and scintillation.
Systems like the Brilliancescope™ and I-see2™attempt to analyze a diamonds contrast patterns with a camera, but try as they might, such systems can not completely account for human perception. Try this experiment; hold a diamond very still and count the bright flashes with one eye closed, then open both eyes and count again. Two eyes can see as many as twice the number of contrasting bright flashes than one eye (or a camera lens).
If a bright facet has darker adjoining facets, the bright facet will have good contrast and appear even more brilliant, like the lower left pattern in Fig1. If there is very little contrast between adjacent facets then there will be little sparkle, like the lower center and right side in Fig 1, even though the overall light return may be the same.