Dilithium crystal
The dilithium crystal is the most valuable gem in NetHack, with a base value of 4500 zorkmids. It is white and soft (cannot engrave in the ground).
It starts appearing when you have reached at least dungeon level 27.
SLASH'EM
In slashem, it can not be used in gem alchemy by dipping it in acid. It always explode instead.
Origin
It's an important fictional mineral from Star Trek.