Cut

The cut is arguably the most important "C" of the four Cs; it determines the shape and beauty of a diamond. The breathtaking sparkle of a diamond is almost entirely due to the cut of a diamond. 

Diamonds must be cut in a way that allows light to reflect off of the tiny facets in the stone.

If a stone is cut too shallow, light escapes out the bottom, and if a diamond is cut too deep, light escapes through the side. However, when the diamond is the perfect proportion, light reflects brightly. This light reflection is what makes a diamond sparkle!