Thulium

Thulium is a soft, silvery lanthanide metal that slowly tarnishes in air. It is never found pure in nature; small amounts occur mixed with other rare-earth elements in minerals such as monazite, xenotime, and gadolinite. To obtain it, producers separate thulium from the other rare earths in processed ore, then convert its compounds into the metal. Thulium is used sparingly because it is rare and costly. A memorable application is in certain lasers, where a small amount of thulium is added to the laser material to help it produce light. Radioactive thulium-170, rather than ordinary metallic thulium, can also serve as an X-ray source in some portable devices.