magnetite, lustrous black, magnetic mineral, Fe₃O₄. It occurs in crystals of the cubic system, in masses, and as a loose sand. It is one of the important ores of iron (magnetic iron ore) and is a common constituent of igneous and metamorphic rocks. It is found in Norway, Sweden, the Urals, and various parts of the United States. A variety of magnetite, lodestone (or loadstone) exhibits polarity and is especially interesting for its natural magnetism; Magnet Cove, Ark., became famous as a source.