Pyrite

Hardness: 6-6.5 on Mohs scale. Color: Pale brass-yellow, can tarnish to a dull brassy, or even dark brown or black. Often exhibits iridescent tarnish (blues, reds, purples, yellows) due to oxidation. Luster: Metallic. Crystal Structure: Isometric (cubic), commonly found as cubes, octahedra, pyritohedra, or in massive, granular, or stalactitic forms. Cleavage: Indistinct. Specific Gravity: 4.9-5.2.

Pyrite forms in a wide variety of geological environments, both igneous, metamorphic, and sedimentary. It commonly crystallizes from hydrothermal fluids, in contact metamorphic deposits, and as an accessory mineral in igneous rocks. In sedimentary environments, it can form in anoxic conditions, often replacing organic materials. Its formation age spans billions of years, from ancient Precambrian deposits to modern-day anoxic sediments.

Historically, pyrite was used as a source of sulfur for the production of sulfuric acid, especially during times when sulfur was scarce. It was also used in early radios (cat's whisker detectors). Today, its industrial uses are limited, but it is sometimes used as an ore of iron where other iron sources are not available, or occasionally for its trace element content (e.g., gold, cobalt, nickel) if present in economic quantities. It is popular among mineral collectors and is sometimes cut and polished as a gemstone for ornamental purposes, though its brittleness limits its use in jewelry.

Pyrite is often mistaken for gold due to its metallic luster and yellowish hue, earning it the nickname 'fool's gold.' However, it's easily distinguished by its brittleness (gold is malleable) and its greenish-black streak (gold has a yellow streak). Pyrite is also known for forming perfect cubic crystals, some of which can be quite large and are highly sought after by collectors. In some ancient civilizations, polished pyrite was used as mirrors.

In the field, pyrite can be identified by its metallic luster, brass-yellow color (though often tarnished and iridescent), and its hardness (it can scratch glass). A key identifying feature is its greenish-black streak when rubbed on an unglazed porcelain plate. Unlike gold, it is brittle and will break or crush under pressure rather than deform. It often occurs as well-formed crystals. Pyrite is one of the most widespread sulfide minerals and can be found globally in various rock types. Notable localities include Spain (famous for cubic crystals), Peru, Italy, and various locations in the USA. For collectors, well-crystallized specimens, especially those exhibiting iridescence, are highly valued.