Marcasita

Marcasita, also known as iron pyrite or fool's gold, is a sulfide mineral with the chemical formula FeS2. It is commonly found in coal beds, igneous rocks, and metamorphic rocks. Marcasita has a pale brass-yellow color and a metallic luster, which often leads to its confusion with gold. However, it is much harder and more brittle than gold, and will produce a greenish-black streak when scratched on a porcelain plate, whereas gold produces a yellow streak.

The crystal structure of marcasita is orthorhombic, and it typically forms as tabular or prismatic crystals, often in radiated or botryoidal aggregates. It is polymorphous with pyrites, meaning they share the same chemical formula but have different crystal structures. While both are FeS2, pyrites crystallizes in the cubic system. Marcasita is generally less stable than pyrites and can alter to pyrites under certain conditions.

Historically, marcasita was used in jewelry and ornamentation, especially during the Georgian and Victorian eras. It was often cut and polished and set into items like brooches, pendants, and belt buckles. However, its tendency to tarnish and oxidize made it less durable for long-term wear. Today, marcasita is primarily of mineralogical interest and is collected by hobbyists. It can also be found in association with other sulfide minerals like chalcopyrite and sphalerite, and is sometimes an indicator of hydrothermal activity.