Pyrite

Hardness: 6-6.5 (Mohs scale); Color: Pale brass-yellow to golden; Luster: Metallic, often bright; Crystal structure: Isometric (cubes, pyritohedrons, octahedrons); Cleavage: Indistinct; Specific Gravity: 4.8-5.0

Pyrite forms in a wide variety of geological environments, including hydrothermal veins, sedimentary rocks (often as concretions or replacements of organic matter), and metamorphic rocks. It forms at both high and low temperatures and is common in oxygen-deficient environments.

Historically, pyrite was a minor ore of iron and sulfur. Today, it is occasionally cut and polished as a gemstone or ornamental stone. Due to its metallic luster, it's sometimes used in jewelry, though its brittleness limits its use. It is a source of sulfur for sulfuric acid production, particularly in areas where native sulfur deposits are scarce. Collectors value well-formed crystals.

Pyrite is famously known as "Fool's Gold" due to its superficial resemblance to gold, especially to inexperienced prospectors. However, it is much harder than gold, more brittle, and leaves a greenish-black streak (gold leaves a yellow streak). When exposed to air and water, especially in old mine workings, pyrite can oxidize to form sulfuric acid and iron oxides, a process known as acid mine drainage.

In the field, pyrite is identified by its metallic luster, brassy yellow color, high hardness (cannot be scratched with a nail), and the lack of malleability (it will spark when struck with steel, unlike gold). It often forms distinct cubic or dodecahedral crystals. It is found globally across various geological settings, including Spain (famous for large cubes), Peru, North America (e.g., Colorado, New York), and Italy.