Marcasite

Marcasite is the name given the mineral iron sulfide, which is mined all over the world. However, most of the gem called marcasite used in jewelry is actually iron pyrite, since genuine marcasite often crumbles into dust and is thus too delicate to be used in jewelry. The composition of iron sulfide and iron pyrite are the same, however they crystallize differently. Iron pyrite is commonly referred to as fool's gold, because throughout history many gold miners believed they had struck it rich when encountering iron pyrite with bright, brassy streaks.