Closed culet

Closed Culet - A culet on a diamond that is too small to be resolved with the unaided eye and that can be seen only with difficulty under 10x.

Closed Culet
A culet on a diamond which consist a point rather than a facet. That point is too small to be resolved with the unaided eye and can be seen only under 10X magnification.
Cloud ...

Closed Culet
A culet which consists of a point rather than a facet.
Club ...

Any sharp meeting place of three or more facets, such as a corner, or a closed culet; forms a weak point in polished diamonds, as a sharp blow to a point could easily cause the diamond to cleave (break).

See also: Pink, Cloud, Rough, Culet, Clarity