marcasite (n.) Look up marcasite at
crystalized pyrite, early 15c., from Medieval Latin marchasita, of obscure origin, perhaps via Spanish, probably from Arabic, though OED doubts this. Perhaps ultimately from Persian marquashisha [Klein]. "This name has been used for a number of substances but mainly for iron pyrites and especially for the crystalline forms used in the 18th century for ornaments." [Flood]