crystal: [OE] The prehistoric Indo-European base *kru- produced several words denoting ‘hard outer surface’, including English crust, Old High German hrosa ‘crust’, and Old Norse hrúthr ‘crust’. In some cases they reflect a hardening caused by freezing: Old High German hrosa, for example, also meant ‘ice’, and Greek krúos meant ‘frost’.
From this was derived krustaímein ‘freeze’, which in turn formed the basis of krústallos ‘ice’. When Old English first acquired the word, via Latin crystallum and Old French cristal, it still meant ‘ice’, a sense which survived until the 16th century, although losing ground all the time to the metaphorical extension ‘clear mineral’. => crust
crystal (n.)
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.