Pyrite

Hardness: 2.5–3 (Mohs), Color: Golden yellow to brassy yellow, can tarnish to dark brown or black. Luster: Metallic, very bright. Crystal Structure: Isometric (cubic). Often found as cubes, octahedra, or pyritohedrons, but also massive, granular, or stalactitic. Streak: Greenish-black. Cleavage: Indistinct. Specific Gravity: 4.9–5.2.

Pyrite forms under a wide range of conditions, both igneous, metamorphic, and sedimentary. It is commonly found in hydrothermal veins, in contact metamorphic rocks, as an accessory mineral in igneous rocks, and in sedimentary rocks (especially shales, coal beds, and limestones) where it forms in anoxic environments. Its formation spans geological ages, from Precambrian to recent deposits.

Historically, pyrite was used as an ignition source in early firearms (flintlock mechanisms). Today, it is primarily a source of sulfur for sulfuric acid production, which is crucial for various industrial processes including fertilizers, chemicals, and detergents. It also has minor use as an ornamental stone and in jewelry, sometimes cut into cabochons or beads, though its brittleness limits its use. It is collected by mineral enthusiasts.

Pyrite is often called "Fool's Gold" due to its superficial resemblance to gold. This has led many inexperienced prospectors astray. Despite this, pyrite is an important ore mineral, particularly for sulfur. The name "pyrite" comes from the Greek word 'pyr' meaning 'fire', because it produces sparks when struck by iron or other hard materials. It's also known as "Iron Pyrite."

In the field, pyrite can be identified by its distinctive metallic luster and brassy yellow color (though gold is softer and has a yellow streak). Its cubic or anhedral crystal forms can also be characteristic. Its hardness is greater than that of gold, and it will not easily scratch with a knife (gold is much softer). It commonly occurs in ore deposits worldwide, including Spain (famous for large crystals), Peru, the USA (especially Colorado and New York), and various European countries. When collecting, be aware that pyrite can react with oxygen and water over time to form sulfuric acid, which can damage other minerals in a collection if not stored properly; some specimens may develop a white powdery coating.