Marcasite jewellery is made from pyrite, not from the mineral marcasite. In the late medieval and early modern eras the word “marcasite” meant both pyrite and the mineral marcasite (and iron sulfides in general). The narrower, modern scientific definition for marcasite as orthorhombic iron sulfide dates from 1845. The jewellery sense for the word pre-dates this 1845 scientific redefinition. Marcasite in the scientific sense is not used as a gem due to its brittleness.