Iridium is one of the six platinum-group metals (PGM), the others being platinum, palladium, rhodium, ruthenium and osmium. It is one of the densest naturally occurring elements, with a specific gravity of approximately 22.6, and one of the most corrosion-resistant. In jewellery use, iridium is encountered primarily as an alloying metal in platinum jewellery, where small percentages improve hardness, working characteristics and durability.

Source and rarity

Iridium is one of the rarest elements in the Earth's crust, with crustal abundance estimated at parts per billion. Most production comes as a by-product of platinum and nickel-copper mining in South Africa (the Bushveld Complex) and Russia (Norilsk and the Ural Mountains). Annual world production is small, in the range of a few tonnes, and the metal commands prices that fluctuate substantially based on industrial demand from electronics, catalysts and specialised alloys.

Use in platinum alloys

Platinum jewellery is alloyed with small percentages of iridium to improve mechanical properties. The most common platinum alloys for jewellery are Pt950Ir50 (95 per cent platinum, 5 per cent iridium) and Pt900Ir100 (90 per cent platinum, 10 per cent iridium). The iridium addition raises hardness, improves casting and working characteristics, and reduces susceptibility to scratching, while preserving the white colour and corrosion resistance that make platinum desirable for jewellery use. Other platinum alloys use ruthenium, palladium or cobalt as the alloying metal, with each producing slightly different working properties.