Thulium · element 69
The rarest stable rare earth, named for the edge of the known world, now shrunk into portable X-ray sources
Key facts
Thulium is the lanthanide most people have never heard of, and there is a fair reason for that. Of all the rare earths with stable isotopes, it is the least abundant in the Earth's crust, and for much of the twentieth century it was so expensive that hardly anyone bothered to find a use for it. Its name reaches for the same sense of remoteness: Thule was the name ancient geographers gave to the farthest land of the north.
Yet thulium is not vanishingly rare. It is more common in the crust than silver, gold or platinum. Its scarcity is relative, measured against the other rare earths, and as separation methods have improved it has quietly found a few jobs it does better than anything else, from surgical lasers to X-ray machines small enough to carry.
Where it is found
Thulium is present at very low concentrations in the same ores as every other lanthanide: monazite, xenotime, bastnäsite and the heavy rare earth clays of southern China. In a typical ore it accounts for well under one per cent of the total rare earth content, so it is recovered only as a by-product when heavier lanthanides such as dysprosium and erbium are being processed.
World production is measured in tens of tonnes a year at most. Because it is always the last fraction out of the separation plant, thulium prices reflect the cost of many extra purification stages rather than any true absence from the ground.
How it was discovered
In 1879 the Swedish chemist Per Teodor Cleve was working through erbium oxide, removing every known component he could. What remained contained two new oxides, one brown and one green. He named the brown one holmia, after Stockholm, and the green one thulia, after Thule. The element in thulia became thulium.
Cleve's thulia was far from pure, and the chemist who did most to fix that was Charles James at the University of New Hampshire. In the early twentieth century he carried out an extraordinary campaign of fractional crystallisation, reportedly thousands of repetitions of the same operation, to obtain clean thulium compounds. Pure thulium metal followed in the 1950s once ion exchange methods were available.
What we use it for
The most practical use is in portable X-ray equipment. Thulium-169, the only natural isotope, absorbs a neutron in a reactor to become thulium-170, which has a half-life of about 128 days and emits X-rays and gamma rays as it decays. A small sealed pellet of thulium-170 can serve as an X-ray source for field medicine, dentistry or industrial inspection where mains power and a conventional X-ray tube are not available.
Thulium doped lasers are the other growth area. Thulium ions in a crystal or fibre emit at around 2 micrometres, a wavelength absorbed strongly by water, and thulium fibre lasers have become an alternative to holmium lasers for kidney stone fragmentation and soft tissue surgery.
Thulium also appears in some high temperature superconductors, in microwave ferrites and in security inks that glow blue under ultraviolet light.
Reading the numbers
Thulium melts at about 1545 °C and boils near 1950 °C, giving it a surprisingly narrow liquid range for a metal with such a high melting point. The boiling point is low compared with erbium and lutetium on either side, a reminder that the lanthanides do not follow a single smooth trend. Its density of 9.32 grams per cubic centimetre is well above iron, in line with its position near the heavy end of the series.
Its configuration [Xe] 4f13 6s2 leaves the 4f shell one electron short of full. Thulium forms +3 ions in almost all its compounds, and like samarium and ytterbium it can also reach a +2 state under strongly reducing conditions, because doing so completes the shell. Its first ionisation energy of about 597 kJ/mol and electronegativity of 1.25 sit at the higher end of the lanthanide range, as expected for a small, heavy atom. With only one stable isotope, thulium-169, its atomic mass of 168.93 is very close to a whole number.
Safety and the environment
Thulium compounds are of low toxicity, and the metal itself is stable enough in dry air to be handled without special precautions, although fine powder is flammable. Thulium-170 sources are radioactive and are used only in sealed, shielded form by trained operators.
Because production is so small, thulium has almost no environmental footprint of its own, but it shares the impact of heavy rare earth mining and separation.
Did you know?
- Thulium is the least abundant of the rare earths that have stable isotopes, yet it is still more plentiful in the crust than silver, gold or platinum.
- Its name comes from Thule, the name ancient Greek and Roman writers gave to the northernmost land they knew of.
- Thulium-170, made by irradiating natural thulium in a reactor, powers compact X-ray sources that need no electricity.
- Charles James is reported to have repeated the same crystallisation step thousands of times to obtain pure thulium compounds in the early 1900s.
- Thulium fibre lasers emitting near 2 micrometres are increasingly used in urology as an alternative to holmium lasers for breaking kidney stones.
More in the lanthanides
The nearest neighbours of thulium in the family, by atomic number.
See all 15 lanthanides on one page, or browse all 118 elements.
Frequently asked questions
What is thulium used for?
Its main uses are in portable X-ray sources based on radioactive thulium-170 and in thulium doped lasers for surgery, especially kidney stone treatment. It also appears in some superconductors, microwave ferrites and ultraviolet fluorescent security inks.
Is thulium the rarest rare earth?
Among rare earths with stable isotopes, thulium is the least abundant in the Earth's crust. Promethium is far rarer because it is radioactive and decays away, but for practical purposes thulium is the scarcest of the stable lanthanides.
Why is thulium so expensive?
It occurs at very low concentrations in rare earth ores and is always the last fraction to be separated, requiring many purification stages. Small demand also means it is produced only as a by-product, so there is no economy of scale.
Where does the name thulium come from?
Per Teodor Cleve named it in 1879 after Thule, the land at the northern edge of the world in Greek and Roman geography. The name reflected the element's Scandinavian discovery and perhaps the sense of having reached the limit of what could be separated.
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Written by Anthony. Updated September 2026. Numerical data from Periodic-Table-JSON (CC BY-SA), cross-checked against the Thulium reference article. Spotted an error? Tell us.