The small flat facet at the base of a brilliant, i.e. the lowest facet of the pavilion, parallel with the table. It is made to minimize splintering and is now often omitted, in which case the sharp point at the bottom is called a 'closed culet'. If larger than normal it is called an 'open culet'. The word is sometimes spelt 'culette' or 'collet'.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson