The cut of a diamond is the only characteristic that is within a human’s control. It is the geometric proportions to which the stone was crafted by the diamond cutter. It must be cut to precise angles, cannot be too shallow or deep, and must be polished to have a mirror finish. In order to reflect light, facets are cut to give the stone a prism-like effect.
An ideal cut is a specific cut in which the light will enter the diamond to be reflected and dispersed through the top and not the sides or the bottom. It will have good symmetry and the facets will be well aligned. It will be well polished, have a girdle that is not to thin, and a crown that is not to shallow or deep. Most importantly, the diamond will be cut to show its true brilliance.
Many diamonds will come with a diamond grading report, which is created by an independent gemological laboratory. The Gemological Institute of America does not evaluate a diamond’s cut, however the American Gem Society has it’s own rating scale which ranges from 0 (ideal) and 10 (poor). It’s a good idea to find out from your jeweler if the diamond you are interested in has been graded to determine the actual cut.