The diamond culet is the small area at the bottom of a diamond (bottom of the pavilion, to be exact). It can be a point or small facet parallel to the table.
The culet is a segment of the cut grade. When a diamond is in the grading process, the gemologist views the diamond face up to view the culet’s size, and from the side to determine the culet angle. The culet size can be expressed as a percentage, ranging from 1.5% to 15%. If the culet is sharply angled, it is considered an extra facet instead of a culet.
Culet is graded by the GIA as None, Very Small, Small, Medium, Slightly Large, Large, or Very Large.
If a culet is Medium or smaller, it is most likely invisible to the naked eye and does not negatively impact the diamond quality. Many diamond cutters intentionally create a culet to minimize the risk of the diamond chipping at the point.
However, a culet of Slightly Large or higher may affect light performance within the diamond. This rating would indicate that the culet allows light to enter from the crown and escape through the culet, instead of reflecting the light out. It would be considered a light leakage, and the culet could even appear as an inclusion or dead area when the light escapes.