Thulium was discovered in 1879 by the Swedish chemist Per Teodor Cleve, who drew it from the residues of erbium ore. He named it after Thule, the name the ancient Greeks and Romans gave to the northernmost land at the edge of the known world. Thulium is the rarest of the stable rare-earth metals, soft and silvery, used in portable X-ray sources and in certain surgical lasers. Rare and expensive, thulium has few uses: a radioactive isotope powers small, portable X-ray machines, thulium-doped crystals form the lasers used in some surgery, and its faint blue fluorescence marks certain banknotes under ultraviolet light.
Aside from promethium, which barely exists in nature at all, thulium is among the least abundant of the naturally occurring lanthanides, though at roughly half a part per million it is still about a hundred times commoner in the crust than gold, and its price stays high despite its narrow range of uses. Compact thulium-170 X-ray units, self-contained and drawing on the isotope's own decay rather than any battery, were developed for military field medicine and industrial inspection work in remote locations during the twentieth century, valued precisely because they needed no external power source to generate usable radiation for imaging. Such portable units have since been largely displaced by digital and battery electronic X-ray sources, leaving thulium's isotope role a mostly historical footnote.