Osmium (Os, element 76, CAS 7440-04-2) is a hard, brittle, bluish-gray transition metal and the densest naturally occurring element, at 22.587 g/cm³ (20 °C). It is also the smallest market among the platinum group metals: the world produces an estimated 500-550 kg a year, according to SFA (Oxford) for the International Platinum Group Metals Association (IPA) in May 2026.
Osmium's identity, density, supply, discovery, uses, isotopes, toxicity and rarity follow, each with its source.
What Is Osmium?#
Osmium is a platinum group metal in period 6, group 8 of the periodic table, with the symbol Os and atomic number 76. It sits below ruthenium, between rhenium and iridium, and belongs to the heavy triad of the platinum group metals with iridium and platinum. Osmium is a chemical element, with its standard atomic weight set by the IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW).
Osmium Identity: Symbol, Atomic Number and CAS Number#
Osmium's identity numbers are symbol Os, atomic number 76, CAS 7440-04-2 and standard atomic weight 190.23(3), one of the least precisely known atomic weights of the six platinum group metals. The table lists the identity data.
| Property | Value |
|---|---|
| Symbol | Os |
| Atomic number | 76 |
| CAS Registry Number | 7440-04-2 |
| Standard atomic weight (CIAAW, since 1991) | 190.23(3) |
| Period / group | 6 / 8 |
| Ground-state electron configuration | [Xe] 4f¹⁴ 5d⁶ 6s² |
| Crystal structure (20 °C) | Hexagonal close-packed, a = 273.42 pm, c = 431.99 pm |
The uncertainty of ±0.03 on osmium's atomic weight is about 16 times larger, in relative terms, than iridium's 192.217(2). Two causes explain the gap: osmium's isotopic composition varies in nature, and its isotope osmium-187 forms from the decay of rhenium-187. CIAAW therefore flags osmium for natural variation and has kept the value unchanged since 1991.
Osmium Quick Facts#
Osmium's headline facts are its record density (22.587 g/cm³), its 1803 discovery alongside iridium, and its lack of any dedicated primary producer worldwide. The five core facts are summarized below.
| Property | Value | Source |
|---|---|---|
| Density at 20 °C | 22.587 g/cm³ (22,587 kg/m³) | Crystallographic value, Wikipedia/CRC lineage |
| Melting point | 3,033 °C (3,306 K; 5,491 °F), handbook value | CRC Handbook |
| Common oxidation states | +2, +3, +4, +8 | CRC-derived element data |
| Discovery | 1803, Smithson Tennant | Letter to the Royal Society, 21 June 1804 |
| Primary present-day use | Osmium tetroxide (OsO₄) chemistry | IPA Osmium Factsheet, May 2026 |
Osmium Properties#
Osmium is the densest naturally occurring element and the hardest of the platinum group metals. Its hexagonal close-packed structure also makes it brittle and practically unworkable as bulk metal, so suppliers sell osmium as powder, sponge or alloy additions.
Osmium Density: Why It Is the Densest Element#
Osmium is the densest naturally occurring element, at 22.587 g/cm³ (20 °C), narrowly ahead of iridium's 22.562 g/cm³. The gap is 0.025 g/cm³, or 0.11%, and only X-ray crystallography in the 1990s measured it precisely enough to settle the order. J. W. Arblaster's primary paper, "Crystallographic Properties of Osmium" (Platinum Metals Review, 2013), gives 22.589 g/cm³, which widens the lead to 0.027 g/cm³; either value puts osmium first.
The crystallographic density depends only on the lattice, the atomic weight and the Avogadro constant, so pores and cracks cannot distort it. The worked calculation for osmium's hexagonal cell (2 atoms, a = 273.42 pm, c = 431.99 pm) reproduces the published value:
V(cell) = (√3/2) × a² × c = 2.7969 × 10⁻²³ cm³
m(cell) = 2 × 190.23 g/mol ÷ 6.02214076 × 10²³ /mol = 6.3177 × 10⁻²² g
ρ = 6.3177 × 10⁻²² g ÷ 2.7969 × 10⁻²³ cm³ = 22.588 g/cm³
Osmium stays ahead at every temperature at normal pressure. Arblaster's 2014 paper in Johnson Matthey Technology Review, "Is Osmium Always the Densest Metal?", finds that iridium overtakes osmium only under compression, above 2.98 GPa at room temperature, where both reach 22.750 g/cm³. Legacy Johnson Matthey tables printed the reverse order (iridium 22.65, osmium 22.61 g/cm³), because bulk measurements under-read brittle, porous osmium.
| Property | Osmium | Iridium |
|---|---|---|
| Density at 20 °C (g/cm³) | 22.587 | 22.562 |
| Melting point (°C) | 3,033 (handbook value) | 2,446 |
| Mohs hardness | 7.0 | 6.5 |
| Vickers hardness (MPa) | 4,137 | 1,760-2,200 |
| Discovery | 1803, Tennant | 1803, Tennant |
The osmium vs iridium contest also covers stiffness and hardness; the full comparison is on osmium vs iridium.
Osmium Melting Point and Hardness#
Osmium has the highest melting point of any platinum group metal, 3,033 °C (3,306 K), and is the hardest of the platinum group metals, at Vickers 4,137 MPa, Brinell 3,920 MPa and Mohs 7.0. The 3,033 °C figure is the conventional CRC Handbook value, not a settled constant: the LNG compilation gives 3,045 °C, and Arblaster's 2023 assessment in Johnson Matthey Technology Review works to a melting point near 3,400 K (about 3,127 °C). On the handbook value, osmium ranks third among metals after tungsten (3,422 °C) and rhenium (3,186 °C), and it boils at 5,008 °C (5,281 K); PubChem lists 5,285 K.
Osmium is not stronger than diamond. Diamond's Vickers hardness is about 115 GPa against osmium's roughly 4 GPa, so diamond is about 29 times harder. The comparison that does hold is stiffness: Cynn and colleagues at Lawrence Livermore National Laboratory reported osmium's bulk modulus at 462 GPa in Physical Review Letters (2002), against 443 GPa for diamond. Later work lowers osmium to a 395-462 GPa band, so the "less compressible than diamond" claim remains method-dependent.
Osmium Electron Configuration, Oxidation States and Reactivity#
Osmium's electron configuration is [Xe] 4f¹⁴ 5d⁶ 6s², and its common oxidation states are +2, +3, +4 and +8, within a full observed range of -2 to +8. The +8 state, reached in osmium tetroxide (OsO₄), is osmium's signature; PubChem's headline field lists only +3 and +4 and omits it. Osmium and ruthenium are the only platinum group metals that reach +8.
Finely divided osmium metal slowly oxidizes in air at room temperature to volatile OsO₄, which makes powdered osmium the least air-stable platinum group metal; bulk synthesis of the tetroxide requires heating to 400 °C. Refiners use the same volatility to distill osmium and ruthenium away from the other four platinum group metals as tetroxides. At high temperature the loss is extreme: C. A. Krier and R. I. Jaffee (Journal of the Less-Common Metals, 1963) reported osmium losing mass in 1,400 °C air about 125,000 times faster than platinum.
Where Osmium Comes From#
Osmium is the "orphan" platinum group metal: it is included in no standard producer reporting code (2E, 3E, 4E, 5E+Au or 6E), is generally not paid for in concentrate, and appears in no major company's Mineral Resource statement. Every gram is a by-product of platinum, palladium or nickel ore, recovered, when it is recovered at all, from refinery streams that also yield iridium and ruthenium.
Osmium Ore and Minerals#
Osmium occurs in native osmium (an Os-Ir-Ru alloy) and in the mineral erlichmanite, OsS₂, both tied to chromite-bearing ore alongside iridium and ruthenium. The main host phases are listed below.
- Native osmium, (Os,Ir,Ru): hexagonal, calculated density 22.59 g/cm³ (measured 22.48), found as prismatic inclusions in platinum-iron alloy grains
- Erlichmanite, OsS₂: inclusions in platinum-iron alloy nuggets and in chromite, part of the laurite-erlichmanite series with ruthenium sulfide
- Pyrrhotite and pentlandite: osmium in solid solution, partly in pentlandite in South Africa's UG2 chromitite
The only country split of osmium in the ground, Vermaak's 1995 estimate, quoted in the IPA Osmium Factsheet (May 2026), puts 54% of in-situ osmium resources in South Africa, 28% in Russia, 9% in Finland and 6% in Zimbabwe.
Osmium Supply and Production#
World osmium production is estimated at 500-550 kilograms a year in 2024 and 2025 (about 16,100-17,700 troy ounces), according to SFA (Oxford) for the IPA; no producer publishes a separate osmium reserve or output figure. The same factsheet applies a 0.1-0.5% osmium share to contained PGMs and derives about 76-380 t of osmium in global reserves, with a best estimate near 150 t.
Refinery practice, not ore, limits supply. The IPA factsheet reports that two of the four major South African refineries do not recover osmium, that most osmium in South African circuits is lost before final refining, and that ruthenium purification sends osmium tetroxide to vent scrubbers. Anglo American Platinum (now Valterra Platinum) recovered osmium as potassium osmate until about 1990, Russian recovery stopped by the end of the 2010s, and an osmiridium concentrate from the Evander Kinross gold plant ceased in 2020.
US osmium imports for consumption show the same thinness, according to USGS Mineral Commodity Summaries (February 2026).
| Year | US osmium imports for consumption (kg) |
|---|---|
| 2021 | 1 |
| 2022 | 1 |
| 2023 | 0 |
| 2024 | 1 |
| 2025 (estimate) | 57 |
Osmium Discovery and History#
Osmium was discovered in 1803 by Smithson Tennant, working in London with William Hyde Wollaston; Tennant reported it to the Royal Society, alongside iridium, in a letter dated 21 June 1804. Tennant named the metal from the Greek osme ("a smell") for the odor of its tetroxide, which he described as "chlorine-like and slightly garlic-like".
The source material was the black residue that crude platinum leaves after dissolving in aqua regia. Joseph Louis Proust took that residue for graphite, and the French chemists Collet-Descotils, Fourcroy and Vauquelin proposed the name "ptène" for what they believed was a single new metal. Tennant obtained more residue, fused it with sodium hydroxide, distilled volatile OsO₄ from the acidified solution and showed that the residue held two new elements. The Royal Society awarded him the Copley Medal in 1804.
Osmium Uses, Compounds and Alloys#
Osmium has seven application groups in the IPA Osmium Factsheet (May 2026), from chemical oxidation, the largest, to jewelry, the smallest; nearly all rest on osmium tetroxide chemistry or on hard osmium-iridium alloys.
What Is Osmium Used For?#
Osmium's applications have mostly been applications it lost: incandescent lamp filaments (displaced by tungsten) and a Haber-process catalyst (displaced by iron), leaving electron-microscopy staining and asymmetric dihydroxylation as its main present-day chemical uses. The table traces each use by period and outcome.
| Use | Period | Outcome |
|---|---|---|
| Lamp filaments (Carl Auer von Welsbach's "Oslamp") | 1898, sold from 1902 | Displaced by tungsten; OSRAM (1906) takes its name from osmium and wolfram |
| Haber-process ammonia catalyst (BASF) | c. 1908 | Displaced by iron catalysts |
| Osmiridium pen-nib tips, needles, pivots, contacts | 19th-20th century | Largely displaced by cheaper hard alloys |
| OsO₄ lipid staining for transmission electron microscopy | 20th century to present | Still standard |
| OsO₄ dihydroxylation in fine-chemical synthesis | 20th century to present | Main market (IPA, 2026) |
| Latent fingerprint development | 20th century to present | Still used |
| Pt/Os 90/10 alloy in surgical implants (pacemaker parts, pulmonary valves) | Present | Niche (IPA, 2026) |
| Crystalline osmium collectibles | 2013 process, 2010s to present | Niche; vendor-administered price |
Osmium reaches jewelry mainly as crystallized osmium cut into disks and shapes, and the IPA calls jewelry demand "negligible and largely confined to novelty or niche segments". Refiners such as Heraeus sell osmium metal in solid and powder form, and Ames Goldsmith Ceimig supplies sponge; the osmium rod is one fabricated form, specified on the osmium rod page.
Osmium Compounds#
The compound that defines osmium's public profile is osmium tetroxide, OsO₄, used to stain lipids for electron microscopy and as the oxidant in Sharpless asymmetric dihydroxylation, for which K. Barry Sharpless shared the 2001 Nobel Prize in Chemistry with William S. Knowles and Ryoji Noyori. Potassium osmate is its non-volatile substitute in the AD-mix reagents. The two compounds are compared below.
| Property | Osmium tetroxide | Potassium osmate dihydrate |
|---|---|---|
| Formula | OsO₄ | K₂OsO₄·2H₂O (K₂[OsO₂(OH)₄]) |
| CAS number | 20816-12-0 | 10022-66-9 |
| Oxidation state | +8 | +6 |
| Hazard profile | GHS Danger (H300, H310, H330); UN 2471, Class 6.1, Packing Group I; vapor pressure 7 mmHg at 20 °C | Non-volatile solid; GHS Danger, toxic if swallowed, in contact with skin or inhaled (H301, H311, H331, ChemicalBook data) |
| Primary use | Electron-microscopy stain; Upjohn and Sharpless dihydroxylation | Osmium source in AD-mix dihydroxylation reagents |
Catalytic osmium tetroxide runs at 1-2 mol% with N-methylmorpholine N-oxide as the re-oxidant in the Upjohn dihydroxylation, turning alkenes into syn-1,2-diols. Osmium(iv) oxide, OsO₂ (CAS 12036-02-1), is the non-volatile oxide; both have compound pages, osmium tetroxide and osmium(iv) oxide.
Osmium Alloys#
Osmium's natural alloy with iridium, historically called osmiridium or iridosmine depending on crystal structure, was used for pen-nib tips, phonograph needles and compass pivots before harder synthetic alloys replaced it. Under the 1963 nomenclature, osmiridium is cubic with less than 32% osmium, and iridosmine is hexagonal with 32-80% osmium. Aqua regia dissolves platinum but leaves osmiridium intact, which is why Tennant's residue existed. Tasmania's Adamsfield field produced 34.7 kg of alluvial osmiridium from 1940 to 1945.
Osmium Isotopes and Radioactivity#
Osmium has seven naturally occurring isotopes: five are strictly stable and two, osmium-184 and osmium-186, are very long-lived alpha emitters that behave as stable for practical purposes. The seven isotopes are listed below with CIAAW abundances.
- Osmium-192: 40.78%, the most abundant, stable
- Osmium-190: 26.26%, stable
- Osmium-189: 16.15%, stable
- Osmium-188: 13.24%, stable, the normalizing isotope in rhenium-osmium dating
- Osmium-187: 1.96%, stable, radiogenic from rhenium-187
- Osmium-186: 1.59%, alpha emitter, half-life 2.0 × 10¹⁵ years
- Osmium-184: 0.02%, alpha emitter, half-life 1.12 × 10¹³ years
Rhenium-187 decays to osmium-187 with a half-life of 41.6 billion years, and geologists use that pair to date sulfide ores, such as molybdenite, gold and copper-nickel deposits. Among 31 artificial osmium radioisotopes, osmium-191 (half-life 14.99 days) feeds the generator for iridium-191m used in radionuclide angiocardiography.
Is Osmium Toxic?#
Osmium metal itself carries no listed OSHA, NIOSH or ACGIH exposure limit, but its oxidation product, osmium tetroxide, has a NIOSH and ACGIH exposure limit of 0.002 mg/m³ and can irreversibly stain the cornea, causing blindness. The hazard follows from chemistry already described: finely divided osmium slowly forms OsO₄ in air, and OsO₄ reaches a vapor pressure of 7 mmHg at 20 °C.
The NIOSH Pocket Guide sets the tetroxide's limit at 0.002 mg/m³ (0.0002 ppm) as a time-weighted average, with a 0.006 mg/m³ short-term limit and an IDLH (immediately dangerous to life or health) level of 1 mg/m³, so an open vial at room temperature produces hazardous vapor. Laboratories store OsO₄ in glass under refrigeration because the vapor penetrates plastics.
How Rare Is Osmium?#
Osmium is the rarest platinum group metal by reported output, at an estimated 500-550 kilograms a year worldwide, though no reliable public supply series exists for it. The IPA/SFA factsheets of 2026, which cite Johnson Matthey's January 2026 figures for the other five metals, give the 2025 comparison below.
| Metal | 2025 primary supply (t) |
|---|---|
| Palladium | 197 |
| Platinum | 170 |
| Ruthenium | 30.6 |
| Rhodium | 22 |
| Iridium | 7.2 |
| Osmium | about 0.5 (500-550 kg, SFA estimate) |
On that basis osmium is about 0.12% of platinum group metal primary supply. The crystalline-osmium vendor's own material cites about 1,000 kg a year since 2019, roughly double the SFA estimate.
Explore Osmium#
Osmium's current price and its attribute pages sit on their own dedicated pages, linked below with the other platinum group metals.
Osmium Price Today#
Osmium is priced per gram, not per troy ounce, because it has no exchange-traded futures or benchmark auction. Johnson Matthey, Heraeus and Umicore quote five platinum group metals but not osmium, so the reference quote comes from the Osmium-Institut, one vendor's administered price. That vendor listed crystalline osmium at €2,289.84 per gram and refined Fine-Osmium at €54.30 per gram on 24 September 2026. The live quote is on osmium price today, with its history on the osmium price chart.
Engelhard historically quoted osmium sponge at $300-400 per troy ounce, according to the IPA; the troy ounce converter turns grams into troy ounces.
Osmium Attribute Pages#
Each osmium attribute has its own page, from osmium properties to how rare osmium is, listed below.
| Attribute | Page |
|---|---|
| Physical and chemical properties | Osmium properties |
| Density | Osmium density |
| Melting and boiling point | Osmium melting point |
| Electron configuration and oxidation states | Osmium electron configuration |
| Uses | Osmium uses |
| Occurrence, ores and minerals | Osmium occurrence |
| Discovery and history | Osmium discovery |
| Isotopes and radioactivity | Osmium isotopes |
| Compounds | Osmium compounds |
| Alloys | Osmium alloys |
| Toxicity and health effects | Osmium toxicity |
| Rarity | How rare osmium is |
Other Platinum Group Metals#
Osmium shares its ores and refinery circuits with five other platinum group metals, listed below.
| Metal | Density at 20 °C (g/cm³) | Page |
|---|---|---|
| Platinum | 21.452 | Platinum (Pt), element 78 |
| Palladium | 12.007 | Palladium (Pd), element 46 |
| Rhodium | 12.423 | Rhodium (Rh), element 45 |
| Iridium | 22.562 | Iridium (Ir), element 77 |
| Ruthenium | 12.364 | Ruthenium (Ru), element 44 |
Crystalline osmium as an investment carries no buyback guarantee from its vendor; the case is weighed on osmium as an investment. Scrap that carries platinum group metals is assessed with scrap platinum, palladium and rhodium, and supply and demand for the whole group sit on the platinum group metals market page.
Osmium FAQ#
What is osmium used for?#
Osmium is used today mainly for osmium tetroxide staining in electron microscopy, asymmetric dihydroxylation catalysis and crystalline osmium as a collectible. Smaller uses include fingerprint development and Pt/Os 90/10 implant alloys.
Why is osmium toxic to handle?#
Osmium is dangerous to handle in fine or dissolved form because it oxidizes to osmium tetroxide, a volatile compound with a vapor pressure of 7 mmHg at 20 °C that irritates the eyes and can permanently stain the cornea. Solid osmium metal has no listed exposure limit.
Is osmium radioactive?#
Bulk osmium is not radioactive in any practical sense: two of its seven natural isotopes, osmium-184 and osmium-186, are alpha emitters with half-lives above 10¹³ years. Together they make up about 1.6% of natural osmium.
Is osmium magnetic?#
Osmium is paramagnetic, with the smallest magnetic response of the six platinum group metals (+11 × 10⁻⁶ cm³/mol). No platinum group metal is ferromagnetic in pure form at any temperature.
How rare is osmium?#
Osmium is the rarest platinum group metal: world production is estimated at 500-550 kilograms a year (SFA/IPA, 2024-2025), against 170 t of platinum and 197 t of palladium primary supply in 2025 on the IPA/SFA factsheet basis.
Is osmium the heaviest element?#
Osmium is the densest element, not the heaviest by atomic mass: at a standard atomic weight of 190.23, it is lighter atom-for-atom than platinum (195.084) or gold (196.967), but packs more mass into the same volume.