Thulium

Thulium is a soft, silvery rare-earth metal, and apart from radioactive promethium, the least abundant of them all. Per Teodor Cleve discovered it in 1879 and named it after Thule, an ancient name for the far north. It is extracted from rare-earth minerals and clays, mostly in China.

Because it is scarce and pricey, thulium has a handful of specialist uses. Thulium-170, made in nuclear reactors, gives off X-rays and powers portable X-ray devices for inspecting welds and machinery. Fibers laced with thulium make lasers that cut and seal tissue with little bleeding, and surgeons increasingly use them to break up kidney stones. Thulium is also added to some laser crystals and specialty glasses.