Magnetite. Coloured scanning electron micrograph (SEM) of magnetite from a granite drill sample in Bedfordshire in the United Kingdom. Magnetite is a mineral and one of the three common naturally-occurring oxides of iron. Its chemical formula is Fe3O4, and it is a member of the spinel group. Magnetite is the most magnetic of all the naturally-occurring minerals on Earth. Naturally-magnetized pieces of magnetite, called lodestone, will attract small pieces of iron, which is how ancient peoples first discovered the property of magnetism. Small grains of magnetite occur in almost all igneous and metamorphic rocks. Magnification:x 4000 when printed at 10cm high.