Thulium was discovered in 1879, along with holmium, by Per Teodor Cleve, who named the oxide thulia after an ancient name for Scandinavia. It is found in small amounts in such rare-earth minerals as laterite ionic clays, xenotime, and euxenite and in products of nuclear fission. Thulium is one of the rarest of the rare-earth elements. Its abundance in Earth’s crust is nearly the same as those of antimony and iodine.
