thulium - a soft silvery metallic element of the rare earth group; isotope 170 emits X-rays and is used in small portable X-ray machines; it occurs in monazite and apatite 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. apatite - a common complex mineral consisting of calcium fluoride phosphate or calcium chloride phosphate; a source of phosphorus 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 |