9/3/2010
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In modern round brilliant cut diamonds larger than melee sizes, this is not very common, as most round brilliants are cut from a roughly pyramid shaped piece of rough (half the octahedral crystal) which is usually slightly steeper angled than needed for correct pavilion angles. Thus its far more common to see pavilion angles that are too deep, than too shallow. Those distorted crystals which might tend to produce fisheye proportions are more often cut into fancy shapes, so as to avoid that problem. But you DO see the fisheye look in some of the old european cut stones, and in occasional melee diamonds.
The big gotchas with regard to princess cut proportions is that the usually desired numbers go right out the window... Normally, one wants a 43 percent pavilion in a round brilliant cut, in order to achieve maximum internal reflection, and thus, maximum brilliance. The measurement is usually made as depth of the pavilion divided by the width (narrowest dimension). In practice, its usually determined with an optical comparator, either a GIA proportionscope, or a similar eyepiece reticle used in a microscope. In either case, the number is related by convention to the width of the stone. In princess cuts, however, that narrow dimension, the width, is not the optically important or active direction. Most of the pavilion facets radiate from the corners of the stone, and the sides are simply squared up, with facets that have little effect on overall brilliance. So one usually finds depth percentages that seem to be far too deep. In fact, if you measure those pavilions using a corner.
As the pavilion angles get too deep (above about 43 to 44 percent), the table reflection gets darker, and larger. As this effect increases, the diamond starts to look more like a donut, with a brilliant periphery, and a mostly non-brilliant dark area filling up the table region. If this is so extreme as to really just look like a black table area (sometimes extending to the star facets as well), then its nicknamed a nailhead. While this degree of deep pavilion, like the fisheye degree of shallow pavilion, is not very common (The stones are quite ugly and hard to sell, so few cutters push the limits that far), an intermediate range of deep pavilion is quite common. More so, in fact, than truly well cut stones.
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