Pyrite. Pyrite or iron disulphide (FeS2) is the most abundant sulphide mineral, more commonly known as fool's gold. It is mined world wide as a source of sulphur, which is used to produce sulphuric acid. The word pyrite comes from the Greek, which translates as stone which strikes fire. Iron pyrite often occurs as well-formed cubic, octahedral or pyritohedral crystals. It occurs as an accessory in igneous, contact metamorphic and sedimentary rocks. Although not very valuable, it is prized by collectors for its infinitely repeating geometry.