Fool's gold or Pyrite in a bag, 36 pieces in a beautiful counter display.

Pyrite or iron pyrite is a mineral with the formula FeS2 (chemical name: iron disulfide). The mineral is an important iron and sulfur 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 sometimes also incorrectly used, but this refers to yellow mica or resin. Because pyrite occurs in large quantities, 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 sulfur-containing minerals, but also with oxides, in quartz veins, in sedimentary rocks in coal beds and as a replacement mineral in fossils. Pyrite is a disulfide with sulfur 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 releases sulfuric 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 pyre), because one could make sparks by striking pyrite against flint or iron. Today we understand something completely different by flint (a certain form of SiO2). Other names that one may come across 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.