Pyrite

Hardness: 6-7 (Mohs), Color: Gold-yellow to silvery-white, sometimes iridescent with tarnish. Opaque. Luster: Metallic. Crystal Structure: Isometric (cubic, pyritohedron, octahedron). Cleavage: Imperfect. Specific Gravity: 4.9-5.2.

Pyrite forms under both high and low temperatures, in a wide variety of geological settings. It can precipitate from hot hydrothermal fluids (igneous and metamorphic rocks), form in sedimentary rocks under anoxic conditions where iron and sulfur are present, and also through contact and regional metamorphism. Its geological age varies widely depending on the formation environment.

Historically, pyrite was used in igniters for early firearms. Today, it is a significant source of sulfur for sulfuric acid production and, less commonly, a source of iron. It is sometimes collected as a mineral specimen due to its attractive crystal forms and metallic luster. While not a gemstone, polished pyrite is occasionally used in inexpensive jewelry.

Pyrite is often mistaken for gold, which is how it earned the nickname "Fool's Gold." It is also known as "fire-stone" because it can ignite sparks when struck against metal, a method historically used in early firearms. Pyrite can form in various geological environments, including hydrothermal veins, sedimentary rocks, and metamorphic rocks. It frequently replaces organic material in fossils.

Pyrite is easily identified in the field by its distinctive metallic luster, brassy yellow to gold color, and characteristic crystal forms (cubes, pyritohedrons, and less commonly octahedrons). Its streak is greenish-black. It is commonly found worldwide in various rock types, often associated with other sulfides, quartz, and other minerals in ore deposits. To differentiate it from gold, remember that pyrite is brittle and crumbles when scratched, whereas gold is malleable. Pyrite is also much harder than gold (6-7 vs. 2.5-3 Mohs).