|Atomic mass||168.9342 amu|
Thulium, is the least abundant of all the naturally occurring rare-earths. 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.