Iron oxides are chemical compounds composed of Iron and Oxygen. Altogether, there are sixteen known iron oxides and oxyhydroxides. Iron oxides and oxide-hydroxides are widespread in nature, play an essential role in many geological and biological processes, and are widely used by humans. Some usage examples are iron ores, pigments, catalysts, in thermite (see the diagram) and hemoglobin. Common rust is a form of iron(III) oxide. Iron oxides are widely used as inexpensive, durable pigments in paints, coatings, and colored concretes. Colors commonly available are in the "Earth" end of the yellow/orange/red/brown/black range. When used as a food coloring, it has E number E172.