Pyrite

Pyrite forms at both high and low temperatures and occurs, usually in small amounts, in igneous, metamorphic and sedimentary rocks worldwide. It commonly forms in oxygen-poor, organic-rich sediments such as coal and black shale, where it can replace plant debris and shells to form pyritised fossils. The International Gem Society lists fine crystals from Leadville (Colorado), French Creek (Pennsylvania), Bingham (Utah), Elba (Italy), Ambassaguas (Spain), England, Austria, Germany, Switzerland, Sweden, Peru and Bolivia; geology.com illustrates cubic crystals from Navajún, La Rioja, Spain.

Pyrite is recognised by its brass-yellow colour, metallic lustre, high specific gravity, hardness of 6 to 6.5, brittleness and greenish-black streak; it is non-magnetic. Gold is much softer, bends or dents under pin pressure, leaves a yellow streak and is far heavier. Marcasite, a dimorph with the same composition but orthorhombic structure, is paler brass, sometimes greenish, more brittle and slightly lighter (specific gravity about 4.8). Jewellery stones traditionally called "marcasite" are usually pyrite, because true marcasite is less stable.

Pyrite is brittle and heat sensitive. Exposed to air and moisture it oxidises and tarnishes, and the oxidation produces acids, so geology.com notes it is not an excellent jewellery stone; keep it dry. Marcasite oxidises even faster, which is why pyrite is used in its place in jewellery.

The name comes from the Greek "pyr", fire, because pyrite throws sparks when struck against metal or another hard material; it was used to start fires and as a spark-producing material in flintlock firearms. Pyrite jewellery, mostly sold as "marcasite", was popular in the United States and Europe in the mid- to late 1800s, including the Victorian era. Pyrite was also once an important ore for producing sulfur and sulfuric acid, and today it is mined in some deposits as an ore of gold that it contains.