Cut
This is the most important factor in choosing any stone - as far as we're concerned. Proportions and cut determine the brilliance of a diamond. In order to maximize brilliance, the diamond cutter must precisely place each of the diamond's facets, which act as light dispersing mirrors, in exact geometric relation to one another. On a classic round brilliant cut diamond 58 facets must be precisely aligned.
Few diamonds are cut to exact standards since diamond cutters try to maximize their returns on the rough material by cutting stones as large as possible with minimum waste. The proportion, symmetry, and perfection of a cut and shape are sometimes reduced to play a secondary role. The result is usually a compromise between profit (size) and beauty (quality of the cut, proportions and shape).
Ideal Cut
The most important criterion of the cut is the ratio of the stone's depth to its diameter. In order for a diamond to fall within the "ideal" proportion the ratio should be between 58% and 62%. The measurements are taken in millimeters. The crown angles, girdle thickness, and symmetry are also important. Each of these proportion criteria have tolerance ranges. However, these are less crucial when violated than deviation from the depth/diameter ratio.
Marcel Tolkowsky is credited with calculating in 1919 the ideal proportions and facet angles to create maximum brilliance and fire in a diamond. Unfortunately the "ideal cut" results in smaller weight yield from the rough diamond crystal and is rarely practiced. Most cutters today slightly compromise Tolkowsky's "ideal cut" in order to achieve better yield from the rough stones.
This is also the case with an Asscher cut diamond. When the diamond is well proportioned, the light path beam is returned directly back to the eye instead of escaping through the bottom or sides of the cut diamond. As a result, the diamond will have better brilliance, sparkle, and fire.
As you can see on the diagram at left there are many ways
for diamond cut to be distorted. For example slanted crown
where the table is not parallel to the girdle.
