The cut of a diamond is the most important characteristic of the "4Cs" and the hardest to judge. It doesn't refer to the diamond's shape, but rather its reflective qualities. The proportions of a diamond, as well as its polish and precision of faceting, determine the sparkle, fire and brilliance of a diamond. The cut can also influence a diamond's perceived size and, to some degree, its apparent color.
To maximize a diamond's brilliance, it must be cut in a geometrically precise manner. Above is an example of how a finely cut diamond will reflect light. The most noticeable features of the cut are symmetry, polish and faceting; however, equally important are percentages for depth, height and angles. With properly aligned facets, light will enter the diamond and reflect back through the large top facet. Diamonds that are cut too deep or too shallow leak light through the side or bottom, resulting in less brilliance and value.