Old English cristal "clear ice, clear mineral," from Old French cristal (12c., Modern French crystal), from Latin crystallus "crystal, ice," from Greek krystallos, from kryos "frost," from PIE root *kru(s)- "hard, hard outer surface" (see crust). Spelling adopted the Latin form 15c.-17c. The mineral has been so-called since Old English; it was regarded by the ancients as a sort of fossilized ice. As a shortened form of crystal-glass it dates from 1590s. As an adjective, from late 14c.