Cerium

It is a ductile iron-gray metal, one of the most reactive rare earth elements, easily oxidized in humid air and decomposed in hot water. The heat generated by scratching Cerium in the air can cause it to spontaneously combust. Cerium has some very good environmental uses: it can be used to produce safer red pigments; adding small amounts of cerium to fuels can reduce the amount of pollutant particles produced in the exhaust. Cerium compounds are also used commercially as electrodes in searchlights and high-intensity carbon arc lamps for animation projectors. Cerium oxides are used to polish lenses for cameras and telescopes, and to automatically clean the inside of ovens to prevent cooking residues from accumulating on the walls.