Thulium is a soft, malleable and ductile silvery grey metal which can be cut with a knife, and is the rarest of the rare earth lanthanide series.

Paramagnetic at room temperature, thulium becomes ferromagnetic at 20K, the lowest Curie temperature of the rare earths. Natural thulium has possible uses in ferrites (ceramic magnetic materials) used in microwave equipment.

Thulium occurs in small quantities along with other rare earths in a number of minerals, being commercially obtained from monazite which contains about 0.007% of the element. The metal is reasonably stable in air, but should be protected from moisture in a closed container.

Like all lanthanide rare earths, thulium is predominantly trivalent, bit also forms divalent compounds. Thulium oxide is a greenish white colour, whilst its salts are bluish green.

As with other lanthanides, thulium has a low to moderate toxicity. It should be handled with care.

Natural thulium consists of just one stable isotope, thulium-169. When this is bombarded in a nuclear reactor, it can be used as a radiation source in portable X-ray equipment. Thulium-171 is potentially useful as an energy source. A total of 29 radioactive isotopes are known, ranging from the proton dripping thulium-147 to the beta decaying thulium-180.

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