Where it comes from
Spirecrawlers, ceiling stalkers and Vaultmaws always leave it. Veilgrips do too, hatchlings and capsules alike, and so do the shoreclaws of the water. Cave pouncers often have a plate from their backs.
A creature crushed to death leaves less.
What it is for
Chitin and charcoal are ground into black powder at the Powder bench, and the Gun table uses it for plasma cells.