ytterbium - a soft silvery metallic element; a rare earth of the lanthanide series; it occurs in gadolinite and monazite and xenotimemetal, metallic element - any of several chemical elements that are usually shiny solids that conduct heat or electricity and can be formed into sheets etc. gadolinite, ytterbite - a mineral that is a source of rare earths; consists of silicates of iron and beryllium and cerium and yttrium and erbium monazite - a reddish-brown mineral containing rare earth metals; an important source of thorium and cerium xenotime - a brown-to-yellow mineral that is a phosphate of yttrium in crystalline form |