Despite its high price and rarity, thulium is used as a dopant in solid-state lasers, and as the radiation source in some portable X-ray devices.

In 1879, the Swedish chemist Per Teodor Cleve separated two previously unknown components, which he called holmia and thulia, from the rare-earth mineral erbia; these were the oxides of holmium and thulium, respectively.

Pure thulium metal has a bright, silvery luster, which tarnishes on exposure to air.

Reactions are slow at room temperature, but are vigorous above 200 °C: Thulium dissolves readily in dilute sulfuric acid to form solutions containing the pale green Tm(III) ions, which exist as [Tm(OH2)9]3+ complexes:[13] Thulium reacts with various metallic and non-metallic elements forming a range of binary compounds, including TmN, TmS, TmC2, Tm2C3, TmH2, TmH3, TmSi2, TmGe3, TmB4, TmB6 and TmB12.

This reaction results in hydrogen gas and Tm(OH)3 exhibiting a fading reddish color.

[22] Thulium was discovered by Swedish chemist Per Teodor Cleve in 1879 by looking for impurities in the oxides of other rare earth elements (this was the same method Carl Gustaf Mosander earlier used to discover some other rare earth elements).

][9][24][25][26][27][28][29] Thulium was so rare that none of the early workers had enough of it to purify sufficiently to actually see the green color; they had to be content with spectroscopically observing the strengthening of the two characteristic absorption bands, as erbium was progressively removed.

The first researcher to obtain nearly pure thulium was Charles James, a British expatriate working on a large scale at New Hampshire College in Durham, USA.

[30] High-purity thulium oxide was first offered commercially in the late 1950s, as a result of the adoption of ion-exchange separation technology.

However, Australia, Brazil, Greenland, India, Tanzania, and the United States also have large reserves of thulium.

Newer ion-exchange and solvent-extraction techniques have led to easier separation of the rare earths, which has yielded much lower costs for thulium production.

In these, where about two-thirds of the total rare-earth content is yttrium, thulium is about 0.5% (or about tied with lutetium for rarity).

[10] Holmium-chromium-thulium triple-doped yttrium aluminium garnet (Ho:Cr:Tm:YAG, or Ho,Cr,Tm:YAG) is an active laser medium material with high efficiency.

[36] The wavelength of thulium-based lasers is very efficient for superficial ablation of tissue, with minimal coagulation depth in air or in water.

[37] Despite its high cost, portable X-ray devices use thulium that has been bombarded with neutrons in a nuclear reactor to produce the isotope Thulium-170, having a half-life of 128.6 days and five major emission lines of comparable intensity (at 7.4, 51.354, 52.389, 59.4 and 84.253 keV).

These radioactive sources have a useful life of about one year, as tools in medical and dental diagnosis, as well as to detect defects in inaccessible mechanical and electronic components.

[38] Thulium is also similar to scandium in that it is used in arc lighting for its unusual spectrum, in this case, its green emission lines, which are not covered by other elements.

[43] The blue fluorescence of Tm-doped calcium sulfate has been used in personal dosimeters for visual monitoring of radiation.

[9] When injected, thulium can cause degeneration of the liver and spleen and can also cause hemoglobin concentration to fluctuate.

[45][46] In humans, thulium occurs in the highest amounts in the liver, kidneys, and bones.