Thulium Of all the naturally occurring rare-earths , thulium is the least abundant. The only rare-earth that is any rarer is promethium because it never occurs in nature. Thulium is a soft malleable ductile and silvery element of the lanthanide series. It occurs in association with yttrium in a number of ores such as xenotime and euxenite found in monoazite sands. Thulium has few important uses.
Thulium produces an isotope that emits x-rays.
Small pellets can be used as a portable source of diagnostic x-rays for medical use. Another application of thulium is in magnetic ceramic materials.