Pyrite. Credit: Mineral Information Institute.
Pyrite, also known as iron pyrites (Fe2S, iron (II) disulfide), is a hard, brittle, brass-yellow sulfide mineral sometimes called fool's gold because of its metallic lustre and resemblance to gold. It occurs worldwide, sometimes in coal, and is a major ore of sulfur. It was formerly used to produce sulfuric acid.
Pyrite crystallizes in the isometric system, usually as cubes. It alters to goethite and limonite. Hardness: 6.5; relative density 5.0.