Cut
While cut does include shape, in terms of the 4 Cs it refers to the proportions of the cut.
A diamond’s cut grade is an objective measure of a diamond’s light performance, or the amount of sparkle a diamond has.
A diamond’s overall proportions, as well as the size and position of its facets, make up the cut. The consistency and balance of these can greatly affect how the stone captures light and reflects it back to the eye. When a diamond is cut with the proper proportions, light enters the diamond and is returned through the top of the diamond. If a diamond is too shallow, light will escape from the bottom of the stone. If it is cut too deep, light will escape out the sides.
Studies have been conducted to find the optimum proportions of a diamond’s cut so that it has the greatest amount of sparkle. If its cut falls within these parameters, it is considered an ideal cut. Diamonds with fine proportions, symmetry and polish optimize their interaction with light and have increased brilliance, dispersion, and scintillation.