Ancient Greek and Roman cartographers described a place called Thule, the most northerly location known to them. Today we think they might have meant Scandinavia, Iceland, or perhaps the island groups like the Shetlands, Orkneys, or Faroes. Later the term Ultima Thule emerged and it came to mean anyplace far beyond the known world. I used to read Hal Foster’s comic strip Prince Valiant when I was a kid. Val, if you know the story, is from Thule.

Thulium (Tm) is a lanthanoid and with the exception of radioactive Promethium (Pm, #59) is the rarest of the so-called rare-earths. A mere 50 metric tons of thulia (thulium oxide, Tm₂O₃) is produced worldwide each year. The main source is monazite sand, a reddish-brown phosphate mineral found in placer deposits. Thulium is used in lasers and semiconductors.

Thulia was first isolated in 1879 by a Swedish chemist named Per Teodor Cleve. A pure sample of thulium metal was not prepared until 1911.