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.