Magnetite

Magnetite (Fe3O4) is the most magnetic mineral occurring naturally on Earth, an iron oxide belonging to the spinel group alongside magnesioferrite, franklinite, and jacobsite. Black to iron-black with a metallic luster and black streak, it typically forms sharp octahedral crystals, sometimes twinned on the {111} plane, and more rarely as rhombic dodecahedra or the unusual cubic habit. Naturally magnetized pieces are known as lodestone, prized since antiquity for attracting iron filings and used in the earliest compasses.

Classic collector crystals come from alpine-vein localities such as Binntal in Switzerland and Val d'Ala in Italy, prized for striated, lustrous octahedra. South America's Cerro Huañaquino near Potosí, Bolivia, has become world-famous for brilliant, perfectly formed octahedral crystals on albite matrix, while Balmat, New York, is celebrated for its rare, brilliant cubic magnetite unlike any other locality. Other important sources include Kovdor on Russia's Kola Peninsula, Nordmark in Sweden, Franklin and Ogdensburg in New Jersey, the Tilly Foster Mine in New York, and Parachinar, Pakistan, each yielding distinctive crystal habits and matrix combinations.

Common associates include hematite, pyrite, chalcopyrite, epidote, calcite, and chlorite-group micas, and magnetite is frequently replaced pseudomorphically by hematite to form martite. Collectors seek fine magnetite for its striking metallic luster, geometric octahedral perfection, genuine magnetism you can test with a hand magnet, and the historical fascination of true lodestone specimens.