Octahedral crystals of Magnetite, here on a volcanic bedrock. Magnetite is a naturally occurring iron oxide (Fe3O4) that is one-part wustite (FeO) and one-part haematite (Fe2O3). It is a member of the spinel group of minerals and the strongest magnetic mineral on Earth. Sometimes referred to as lodestone. It is isometric, with crystals usually having an octahedral habitat and a brownish black metallic lustre. Sample is from Cerro Huanaquino in Bolivia and overall length of sample is 190 mm.