Magnetite, a magnetic mineral form of Iron (II, III) oxide, one of several iron oxides.
A piece of magnetized magnetite that was used as an early form of magnetic compass.
Only magnetite with a particular crystalline structure can act as a lodestone, and then only if it has been exposed to a magnetic field. In nature this may have been due to lightning strikes. The ability to use a lodestone as a compass is thought to have been discovered in China in the 12th century.
