This is the single most important determinant of a diamond's beauty, and the one element that is directly affected by human skill and expertise. More than any other quality, cut determines the fire and brilliance of a diamond.

In order to maximize this fire and brilliance, the diamond cutter must place each of the stone's facets and angles in exact geometric relation to one another. On a classic round brilliant-cut diamond, fifty-seven or fifty-eight facets must be precisely aligned. When a diamond is cut too deep or too shallow (as is most often the case), light leaks out of the bottom of the diamond instead of reflecting back to you. This explains why some diamonds are so much more brilliant than others.

The cut also determines the shape of a diamond. The most common diamond shapes are the round brilliant, the marquise, the pear, the oval, the emerald, and the square.