Thulium is the less abundant of the rare-earths (excluding promethium). However, it is sixtieth fifth in abundance among the constituents of the earth crust. It is, in fact, more abundant than some of the most common elements as cadmium, silver, indium, palladium, platinum, gold, selenium, etc.

The most important sources of thulium are the minerals rich yttrium: xenotime, gadolinite, euxenite, samarskite, fergusonite, blomstrandine, loparite and yttroparisite. However, small amounts of thulium can be found in many common rocks and minerals like apatite and monazite, that also contains cerium. The minerals rich in yttrium are occasionally found in pegmatites in spite of being more frequent when associated to monazite.

Xenotime occurs in Norway, Sweden, Brazil, Switzerland, Malaysia, Korea, North and South Carolina and in Colorado. Gadolinite can be found in Sweden, Norway, Madagascar, Texas, Arizona and Colorado. Euxenite occurs in Finland, Brazil, Norway, Australia, Madagascar, North Carolina and Idaho. Fergusonite is found and extracted in Greenland, Sweden, Norway, North and South Carolina and in Texas. Samarskite occurs in Madagascar, North Carolina and Ontario. Blomstrandine in Norway, luparite and yttroparisite in Russia. Of these minerals, the xenotime and the gadolinite are the ones most easily treated through chemical reaction, originating the pure concentrates of rare-earths. Euxenite is also an important thulium source due to its role in the uranium, thorium and yttrium extraction.