Iron (II) oxide (FeO) The powder in particular can cause explosions as it readily ignites. Colour black.
Iron (III) oxide, (Fe2O3) known in its natural state as Haematite, but also purified for use as a coating in magnetic audio and computer media, where it is known as Ferric oxide.
Iron (II, III) oxide (Fe3O4), better known as the mineral Magnetite or lodestone. Colour black.