culet

noun2

1. •
   A small, flat face often cut at the base of a brilliant-cut gemstone.

   • A final step is to polish the rough as-grown diamond layer on [the] culet of the designer anvil to smooth the surface and shape the culet so that is suitable for high-pressure experiments. — 2012
2. •
   (historical) A component of armor, consisting of overlapping plates designed to protect the buttocks.