The cut of a diamond primarily refers to the diamond’s proportions and the quality of craftsmanship pertaining to the facets of the diamond. Facets are the flat edges of the diamond intended to draw light into the stone. When the proper proportions are used during the cutting process, light reflects out of the table (the top of your diamond). If the diamond is cut too shallow, you will see very little sparkle as light will simply be lost out of the bottom of the stone.
Likewise if the diamond is cut too deep the light will simply vanish out of the sides. The better the diamond’s cut, the more brilliant the diamond will also be when viewing the stone from the top down.
The most perfectly cut round diamond is called the “ideal cut”. Less than 2% of the world’s faceted round diamonds can be classified as ideal cut. Diamonds that are not ideal cut will lose some light through the bottom or sides of the stone. Many diamonds are intentionally cut with less than ideal proportions for the purpose of maximizing carat weight or to give the diamond a larger appearance.