The Culet of a diamond is the very bottom point located on the pavillion. Often times the culet comes to a complete point but can be finished with a very tiny facet. This is the reason why sometimes you may notice modern brilliant diamonds described as having either 57 or 58 facets.
Culet size is described as:
None or PointedVery SmallSmallMediumSlightly LargeLargeVery LargeExtremely Large
Large culets are not as desirable.
The 4 C's of Diamond Value
Watch the Video (3min)Gemological Institute of America