quartz (n.)

"silicon dioxide," 1756, from German Quarz, Zwarc "rock crystal," from Middle High German twarc, probably from a West Slavic source, compare Czech tvrdy, Polish twardy "quartz," noun uses of an adjective meaning "hard," from Old Church Slavonic tvrudu "hard," from Proto-Slavic *tvrd-, from PIE *(s)twer- "to grasp, hold; hard."