I think the second meaning ("A small central plane in the back of a cut gem.") is just a typo - it's called culet, not cullet. Olaf (talk) 15:04, 28 October 2019 (UTC)
- Appears e.g. on page 962 of Knight's American Mechanical Dictionary (1884), so perhaps just dated. Equinox ◑ 17:18, 23 May 2021 (UTC)