marcasite

英 ['mɑːkəsaɪt; -ziːt] 美['mɑrkəsaɪt]

marcasite (n.): 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]