Pyrite

Hardness: 6-6.5 on Mohs scale. Color: Pale brass-yellow to golden. Luster: Metallic, often bright. Crystal structure: Isometric, commonly forming cubes, octahedra, or pyritohedra, sometimes radiating or intergrown masses. Streak: Greenish-black to brownish-black. Cleavage: Indistinct. Specific Gravity: 4.95-5.10.

Pyrite forms in a wide variety of geological settings, both igneous, metamorphic, and sedimentary environments. It commonly forms in hydrothermal veins, as an accessory mineral in igneous rocks, in contact metamorphic rocks, and as concretions and disseminated grains in sedimentary rocks, especially in anoxic conditions. It can also form during diagenesis in sediments. Geological age widely varies based on the deposit.

Historically, pyrite was a minor ore of iron, and it is still mined for sulfur (used to produce sulfuric acid). It can also be a source of gold when found as 'invisible gold' finely disseminated within the pyrite structure. Although sometimes mistaken for gold, it has no significant value as a precious metal itself. It is occasionally used as a decorative stone in jewelry, particularly in its well-formed crystal habits, but its hardness and tendency to tarnish limit its widespread use.

Pyrite is famously known as "Fool's Gold" due to its metallic luster and brass-yellow color, which often leads to it being mistaken for actual gold. Unlike gold, pyrite is brittle and when struck with steel, it produces sparks (earning it the name 'pyrite' from the Greek 'pyr' meaning fire). Pyrite can weather to form goethite or limonite, and in the presence of water and oxygen, it can oxidize to form sulfuric acid, which can be an environmental concern in mining areas (acid mine drainage).

In the field, pyrite can be identified by its distinctive metallic luster, brassy yellow color (though often with a darker tarnish on weathered surfaces), and its hardness (it cannot be scratched by a fingernail or copper penny, and it will scratch glass). Its common cubic or octahedral crystal forms are also very diagnostic. A simple test involves scratching it on an unglazed ceramic plate to observe its greenish-black streak. It is found globally in almost every type of geological formation, co-occurring with other sulfides, oxides, and quartz. Common locations include Spain (especially for large cubic crystals), Peru, the USA (Colorado, Arizona), and various mining districts worldwide. Collectors value well-formed crystals, particularly large and sharp cubes or radiating aggregates.