marcasite n.

crystallized pyrite, early 15c., originally in medicine and alchemy, from Medieval Latin marchasita (c. 1200 in translations from Arabic), from Arabic marqashīthā "iron sulfide" (though OED doubts this), attested from 9c.; 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]