Iron(II,III) oxide
Iron(II,III) oxide is a chemical compound. It can be used as an iron ore (source of a material) called magnetite.  It is black and brittle.  It can oxidize to red iron(III) oxide after some time.  It is attracted to magnets.  It is a compound that contains iron in two different oxidation states; some of the iron atoms are in a +2 oxidation state and some are in a +3 oxidation state.