beryllium (element) (deleted)
Beryllium is a chemical element with the symbol Be and atomic number 4. It is a steel-gray, strong, lightweight, and brittle alkaline earth metal.
From Wikipedia
Most, if not all uses on Danbooru, are in reference to its use to make the two hollow hemisphere neutron reflectors used in the second demon core criticality accident.
(For the average person, it is an obscure element unlikely to be mentioned.)