Pyrite on shungite found in Bahia/Brazil.

Pyrite or iron Pyrite is a mineral with the formula FeS₂ (chemical name: iron disulphide). The mineral is an important iron and sulphur ore. It often forms well-formed (euhedral) crystals in the shape of a cube with characteristic striations and with a golden sheen. It is also called tinsel or fool's gold (from the English fool's gold) because it was sometimes mistaken for gold. The name cat's gold is also sometimes incorrectly used; however, this refers to yellow mica or resin. Because pyrite is abundant, it can give the illusion of great wealth. However, pyrite is not very valuable (although it can be sold). It is found together with other sulphur-bearing minerals but also with oxides in quartz veins, in sedimentary rocks in coal beds and as a replacement mineral in fossils. Pyrite is a disulphide with sulphur pairs (S2)2 and Fe2+ ions in an octahedral environment in a t2g6 low-spin state. Due to the fact that both ions have a closed configuration, it is a diamagnetic semiconductor. During the weathering of pyrite, iron (hydr)oxides are formed, which also release sulphuric acid. This acid can then react with other minerals and thus lead to the formation of, for example, gypsum, alunite and jarosite. An important source of pyrite is the island of Elba. The name pyrite is very old, derived from the old Indo-European word pyr (fire, cf. for example, pyrotechnics for firework art, or the English pyre (burning pile)), because one could make sparks by striking pyrite against flint or iron. Today, flint is understood to mean something completely different (a specific form of SiO₂). Other names for pyrite are ferrous sulphide, iron sulphide, and the less common words ironblende and iron chert. Pyrite used to be sold as fake gold because it has a golden shine. However, the shine changes when the shiny surface tilts; with gold, the golden shine remains the same. Pyrite is also angular, with sharp edges and hard, while gold is round and soft.