Iron pyrite. Cubic crystal of iron pyrite or sulfide (FeS2), also known as fool's gold, in an unidentified matrix. Iron pyrite is used as a source of sulfur and in the manufacture of sulfuric acid. It is the most abundant sulfide mineral, being found in igneous, metamorphic and sedimentary rocks. Its brass-yellow color and metallic luster lead inexperienced prospectors to believe that they have found precious metal.