Marcasite Tumblestone The mineral Marcasite, sometimes called White Iron Pyrite, is iron sulfide (FeS 2). Marcasite is often mistakenly confused with P yrite , but Marcasite is lighter and more brittle, it is the crystal structure that is the main difference between Marcasite and Pyrite. Although Marcasite has the same chemical formula as Pyrite, it crystallizes in a different crystal system, thereby making it a separate mineral. In jewellery, Pyrite used as a gem is improperly termed "Marcasite". True Marcasite is never used as a gem, due to its brittle and chemically unstable structure. Marcasite is incredibly difficult to polish which makes this tumblestone very rare, measures approx. 3.4 cms. Check out my other items ! Be sure to add me to your favourites list !Please remember if you are making multiple purchases to request an invoice before you pay so I can reduce the postage/shipping costs for you.