iron oxide

Any of various compounds of oxygen and iron, including FeO, Fe2O3, and Fe4O5; often formed in the presence of water; as much as fifty times the volume of pure iron; used as a pigment, with various hues depending on the chemical composition, ranging from yellow to orange, red, and dark brown. Also known as rust. See also Indian red, iron stain, rust jacking. (Bucher, 1996)