Pyrite

Hardness: 6-6.5 (Mohs scale), relatively hard. Color: Pale brass-yellow to golden-yellow, sometimes with a brownish tinge. Often tarnishes to a duller, matte finish. Luster: Metallic, very shiny when fresh, resembling gold. Crystal structure: Isometric; commonly forms cubes, octahedrons, or pyritohedrons, often with striated faces. Also found as granular, massive, or botryoidal forms. Cleavage: Indistinct, conchoidal to uneven fracture. Specific Gravity: 4.9-5.2, feels heavier than most non-metallic minerals of similar size. Streak: Greenish-black to brownish-black.

Pyrite forms under both high and low temperature conditions and is found in a wide variety of geological environments. It can occur in igneous rocks as an accessory mineral, in sedimentary rocks under anoxic conditions, and widely in metamorphic rocks, especially in contact or regional metamorphic zones. It often forms in hydrothermal veins, as concretions in coal and shale, and as replacements of fossils. Its formation is generally associated with areas where iron and sulfur are present, often with organic matter, and in environments with low oxygen. Geologically, it has formed throughout Earth's history, from Precambrian to present-day sediments.

Historically, pyrite was used as a source of sulfur for the production of sulfuric acid, and occasionally as an ore for iron, though it's not a primary iron ore. While not a precious metal, it is sometimes collected for its attractive golden luster and distinct crystal forms. In modern times, it has limited industrial use, though it can be used in some forms of battery production and as a semiconductor in certain electronic applications. It's occasionally cut and polished for costume jewelry due to its metallic sheen, but its hardness and brittleness make it less suitable for fine jewelry.

Pyrite gets its common nickname "Fool's Gold" due to its striking resemblance to gold, which has led many prospectors astray. However, it's easily distinguished from gold by its brittleness (gold is malleable), its hardness (gold is much softer), and its streak color (gold has a yellow streak). Pyrite is dimorphous with marcasite, meaning they have the same chemical composition (FeS₂) but different crystal structures. Pyrite will often oxidize when exposed to air and water, leading to the formation of sulfuric acid and iron oxides, which can cause environmental problems in mining areas (acid mine drainage).

In the field, pyrite can be identified by its characteristic brassy yellow color, metallic luster, relatively high density, and distinctive crystal forms (especially cubes with striations). A key test is the streak test – pyrite will leave a greenish-black streak, whereas gold leaves a yellow streak. It is also brittle and will not be easily scratched by a knife, unlike gold. It is commonly found worldwide in various types of rocks, including igneous, metamorphic, and sedimentary rocks. Significant deposits are found in Spain (Rio Tinto), Peru, various parts of the USA, and China. Collectors often seek well-formed crystal specimens. Due to its potential to weather and release acid, handling and storage in controlled environments are sometimes recommended for long-term preservation of specimens.