These rare-earth oxides are used as tracers to determine which parts of a watershed are eroding. Rare Earth Oxides are: Cerium Oxide, Dysprosium Oxide, Erbium Oxide, Europium Oxide, Gadolinium Oxide, Holmium Oxide, Lanthanum Oxide, Lutetium Oxide, Neodymium Oxide, Praseodymium Oxide, Samarium Oxide, Scandium Oxide, Terbium Oxide, Thulium Oxide, Ytterbium Oxide, Yttrium Oxide.