Iron oxide mineral (FeO), the chief member of one of the series of the spinel group. Minerals in this series form black to brownish, metallic, moderately hard octahedrons and masses in igneous and metamorphic rocks and in granite pegmatites, stony meteorites, and high-temperature sulfide veins. Magnetite, as the name implies, is strongly attracted to a magnet. It is a common constituent of iron ores. Magnetite with an intrinsic magnetic field (a natural magnet) is known as lodestone.