CU stands for culet, the small face at the tip of a cut gemstone. On corundum, the culet is carefully shaped for ideal optics and durability. Culets were historically large but modern cutting minimizes their size. An ideal culet balances light performance with decreased risk of chipping. Cutters precisely align the culet while maintaining symmetry. Shapes include point, kite, and keystone. On rounds, the tiny culet has little effect on sparkle. For shapes like trillion, small culets prevent cleavage down the center point. Corundum’s hardness allows a minimized culet to maximize fire and brilliance.