Thulium is the thirteenth member of the Lanthanide series, being located in the first row below the periodic table of elements. It is a silvery-gray metal with a melting point of 1545 degrees Celsius. It was identified in 1879 by Per Theodor Cleve. It is the rarest non radioactive lanthanide. 

Thulium can be used in surgical lasers, X-ray machines, and a few other medical devices.