CU refers to the culet facet on the bottom tip of nanosital stones. Cutters precisely shape the culet for ideal symmetry, light performance and durability. Culets were historically cut large before trends shifted toward smaller or absent culets. The culet type must balance brilliance and protection. On rounds, the culet has minimal effect on appearance. Pointed shapes demand tiny culets to prevent cleavage. An optimized culet showcases nanosital's flawless optics.