A soft, silvery, malleable, ductile element of the lanthanide series that develops a characteristic green tarnish in air. It occurs naturally with other rare-earth elements in the minerals monazite and bastnaesite and is used to color glass and ceramics yellow, as a core material for carbon arcs, and in metallic alloys. Atomic number 59; atomic weight 140.908; melting point 931°C; boiling point 3,520°C; specific gravity 6.773; valence 3, 4. See Periodic Table.