Iron(III) oxide or ferric oxide is the inorganic compound with the formula Fe₂O₃. It is of one of the three main oxides of iron, the other two being iron(II) oxide (FeO), which is rare, and iron(II,III) oxide (Fe₃O₄), which also occurs naturally as the mineral magnetite. As the mineral known as hematite, Fe₂O₃ is the main source of the iron for the steel industry. Fe₂O₃ is paramagnetic, reddish brown, and readily attacked by acids. Rust is often called iron(III) oxide, and to some extent, this label is useful, because rust shares several properties and has a similar composition. To a chemist, rust is considered an ill-defined material, described as hydrated ferric oxide.