Scandium – 21
Scandium is a soft, light metal that is not a lanthanide, but is included in the rare earth elements because it shares their properties. It is typically refined from thortveitite deposits or extracted from uranium mill tailings.
Added to mercury-vapour lamps that mimic sunlight for indoor / night-time film production. One of scandium’s radioactive isotopes (46Sc) is used as a tracing agent in the process of refining crude oil. Potentially useful for spacecraft construction because it has a higher melting point than aluminum.