The point on the bottom of a diamond's pavilion is called a culet (pronounced que-let or que-lay). During the manufacturing process, the culet is often polished as a flat facet so that it does not get chipped as the other facets are polished. Ideally, the cutter ‘closes' this facet to a point, but sometimes it remains as a small extra facet.
A diamond behaves as a window if opposing facets are parallel, and this is exactly what happens if the culet is too big. You see straight out the hole in the bottom. If you have ever seen a large old cut diamond you probably know what we mean.
The culet size is listed on a cert and your diamond's culet should be pointed (no culet), very small, small or medium because these are not visible to the naked eye.
Large to extremely large culets may be visible to the naked eye and can be treated almost as though they are inclusions.
Large (left) and Medium (right) Culet