Thulium (symbol: Tm, atomic number 69) is the second-least abundant naturally occurring rare earth element, with a crustal abundance of just 0.5 mg/kg. Despite its scarcity and cost, thulium has secured commercial footholds in two high-value applications: solid-state medical lasers and portable X-ray sources — both of which depend on properties no cheaper substitute currently replicates.
What Is Thulium? Key Properties
Thulium sits at position 69 in the periodic table, within the lanthanide series. In pure form it is a bright, silvery-gray metal that can be cut with a knife — Mohs hardness of 2 to 3 — and is both malleable and ductile. It tarnishes slowly in dry air and reacts progressively with water to release hydrogen gas.
Its magnetic behaviour shifts with temperature. Below −241 °C thulium is ferromagnetic; between −241 °C and −217 °C it is antiferromagnetic; above −217 °C it becomes paramagnetic. The trivalent ion Tm³⁺ exhibits a distinctive blue luminescence when chemically excited — the property that underpins its use in euro banknote security printing.
| Property | Value |
|---|---|
| Atomic number | 69 |
| Symbol | Tm |
| Standard state | Solid at room temperature |
| Melting point | 1,545 °C |
| Boiling point | 1,950 °C |
| Density | 9.32 g/cm³ |
| Primary isotope | Thulium-169 (stable) |
| Crustal abundance | ~0.5 mg/kg |
What Is Thulium Used For?
Thulium’s applications are narrow but technically irreplaceable. The element’s commercial footprint is concentrated in lasers, radiation sources, and a small cluster of advanced materials uses.
Medical and Surgical Lasers
Thulium-doped yttrium aluminium garnet (Tm:YAG) lasers operate at around 2,010 nm in the mid-infrared spectrum. At this wavelength, the laser ablates tissue with high precision and shallow coagulation depth — making it well-suited to urological surgery, dermatology, and ophthalmology. The holmium-chromium-thulium triple-doped variant (Ho:Cr:Tm:YAG) lases at 2,080 nm and is used in military rangefinders and meteorological instruments. Thulium fibre lasers — where thulium is doped into optical fibre rather than a crystal — are increasingly used in industrial cutting and welding for their efficiency in the 1.9–2.1 µm band.
Portable X-ray Devices
When thulium-169 is irradiated in a nuclear reactor, it becomes thulium-170 — a radioactive isotope with a half-life of 128.6 days. Thulium-170 emits X-rays across five characteristic energy lines (7.4 to 84.3 keV), sufficient to penetrate metal components and human tissue. The result is a compact, self-powered X-ray source requiring only a small lead cup for shielding. These sources are used in field medical diagnostics, non-destructive testing of inaccessible industrial components, and brachytherapy for cancer treatment.
Other Applications
Thulium’s blue fluorescence under ultraviolet light is used as an anti-counterfeiting marker in euro banknotes. The element has also been used in high-temperature superconductors alongside yttrium and in ferrite ceramics for microwave equipment. Trace additions to high-performance aerospace alloys can improve oxidation resistance at extreme temperatures, though this remains a minor application compared to laser and X-ray uses.
Thulium vs Other Heavy Rare Earths
Thulium sits between erbium and ytterbium in the lanthanide series and shares the heavy rare earth classification with commercially prominent elements including dysprosium and terbium. Like those elements, it is predominantly sourced from ion adsorption clay deposits in southern China rather than from hard-rock primary ore bodies. Its crustal abundance is comparable to lutetium — the rarest stable lanthanide — and approximately four times lower than terbium.
Unlike dysprosium and terbium, which anchor the NdFeB permanent magnet supply chain, thulium has no significant role in magnet alloys. Its commercial demand is therefore smaller and more concentrated: a disruption to the laser or medical device industries would affect thulium demand more directly than broader rare earth market cycles.
What Is Thulium’s Supply Chain?
Thulium occurs in monazite, xenotime, euxenite, and gadolinite mineral deposits. It is never extracted as a primary product — it is recovered as a co-product during the separation of rare earth concentrate, particularly from ion adsorption clay ores. The principal producing region is southern China, where ion adsorption clays carry roughly 0.5% thulium relative to total rare earth oxide content. Production and reserve data is drawn from USGS Mineral Resources and cross-referenced against Shanghai Metals Market (SMM) industrial benchmarks.
Outside China, countries with significant rare earth reserves containing thulium include Australia, Brazil, India, Tanzania, and the United States, primarily in monazite-bearing mineral sands and hard-rock carbonatite deposits. World reserves were estimated at approximately 100,000 tonnes of thulium content as of the early 2000s, though USGS data does not disaggregate thulium separately from total rare earth reserves.
Annual production of thulium oxide has historically been in the range of 50 tonnes per year — a figure that reflects the element’s niche demand profile rather than any supply constraint. High-purity thulium metal (99.9%) has traded between approximately $4,600 and $13,300 per kilogram over multi-decade periods, placing it among the most expensive lanthanides alongside lutetium.
Discovery and Name Origin
Thulium was identified in 1879 by Swedish chemist Per Teodor Cleve while analysing impurities in the rare earth oxide erbia. Cleve isolated two new oxide fractions — brown holmia and green thulia — and named the new element after Thule, an ancient geographic term for the far northern lands associated with Scandinavia and Iceland. A relatively pure thulium oxide sample was first obtained in 1911 by Charles James at New Hampshire College, who required approximately 15,000 fractional crystallisation operations to establish sample homogeneity. The metal itself was first isolated in 1936 by Wilhelm Klemm and Heinrich Bommer.
What Is Thulium Worth? Investment and Market Outlook
Thulium does not trade on commodity exchanges and has no SMM spot price benchmark equivalent to the magnet rare earths. Pricing is negotiated directly between producers and end-users, with Chinese separation facilities setting the primary reference. The element’s value is tied to growth in fibre laser manufacturing — a sector expanding at a compound annual rate driven by industrial automation, medical device proliferation, and defence procurement — rather than to the EV or wind turbine cycles that govern neodymium and dysprosium demand.
For investors tracking the broader rare earth sector, thulium is best understood as a by-product credit within a heavy rare earth separation stream. No listed mining company produces thulium as a primary product. Its commercial significance is likely to grow modestly as thulium fibre laser adoption expands in precision manufacturing and surgical applications.
This article is for informational purposes only and does not constitute investment advice.
What is thulium used for?
Thulium’s primary commercial uses are in solid-state and fibre lasers for medical and industrial applications, and as a portable X-ray source. Thulium-doped YAG lasers operate at around 2,010 nm, making them effective for precise surgical procedures with minimal tissue damage. Irradiated thulium-170 is used in compact X-ray devices for field diagnostics and non-destructive testing.
How rare is thulium compared to other rare earth elements?
Thulium is the second-least abundant naturally occurring lanthanide, with a crustal abundance of approximately 0.5 mg/kg. It is rarer than terbium and dysprosium but similar in abundance to lutetium. Annual production of thulium oxide is estimated at around 50 tonnes — a fraction of output for the magnet rare earths neodymium and dysprosium.
Where does thulium come from?
Thulium is extracted as a co-product from rare earth separation, primarily from ion adsorption clay deposits in southern China. It also occurs in monazite and xenotime deposits in Australia, Brazil, India, Tanzania, and the United States, but is never produced as a primary product from any single mine or facility.
What is the price of thulium?
High-purity thulium metal (99.9%) has historically traded between approximately $4,600 and $13,300 per kilogram, placing it among the most expensive lanthanides. Thulium does not have a publicly quoted spot price; pricing is negotiated between Chinese separation facilities and end-users on a contract basis.
What is a thulium laser used for in medicine?
Thulium-doped YAG (Tm:YAG) lasers operate at approximately 2,010 nm in the mid-infrared spectrum. At this wavelength, the laser ablates tissue with high precision and a shallow coagulation depth, making it well-suited to urological surgery (particularly prostate procedures), dermatology, and ophthalmology. Thulium fibre lasers are also used in industrial cutting and precision manufacturing.
