Q: If a diamond's cut is so important, why aren't all natural diamonds cut to superior standards?
Both the cutter and the seller have strong financial incentives to offer diamonds with inferior cuts. When a rough diamond is cut to superior standards, about 50% of the stone's original weight can be removed in the cutting process. By cutting a diamond too deep and/or too wide, the same piece of rough yields a finished diamond of greater carat weight. Unfortunately, these diamonds are dull, milky or dark in appearance. Usually diamond retailers will sell these lower quality cut diamonds on sale and with out a grading card focusing more on the stones color or overall weight.
The mathematical formula for a round diamond works like this: for a cut with exceptional brilliance (rated "very good" or better), the depth of the diamond will measure between 57% to 65% of the total diameter. In other words, a 1 carat diamond that is 6.5mm wide and 3.9mm deep will display an exceptional return of light to the viewer's eye.