Platinum — The Dense, Hypoallergenic White Metal
Platinum — The Dense, Hypoallergenic White Metal
Properties, alloys, and use in fine jewellery
Platinum is a dense, silvery-white noble metal — symbol Pt, atomic number 78 — used in fine jewellery for its durability, hypoallergenic character, and naturally white lustre that requires no plating to maintain. It is one of the platinum-group metals alongside palladium, rhodium, ruthenium, iridium, and osmium, and is the heaviest metal in routine fine-jewellery use. Premium pricing reflects rarity in the earth's crust, the costs of mining and refining, and the higher fabrication labour required relative to gold work.
Physical and chemical properties
Platinum has a specific gravity of 21.45, nearly twice that of 18-carat gold, and a melting point of 1,768 degrees Celsius — several hundred degrees above gold and high enough to require dedicated platinum-rated torches and casting equipment. Hardness as pure metal is about 4 to 4.5 on the Mohs scale, comparable to gold; jewellery alloys are harder. Platinum is highly resistant to corrosion, oxidation, and most acids, with the exception of aqua regia, and does not tarnish in normal wear.
Crucially, platinum is hypoallergenic for the great majority of wearers because it does not contain nickel and the alloying additions used in fine-jewellery alloys — iridium, ruthenium, cobalt — are themselves rarely allergenic.
Alloys and fineness
Jewellery platinum is alloyed for hardness and workability and is sold in a small number of standard fineness grades. Pt950 — 95.0 percent pure — is the international benchmark for fine platinum jewellery and the UK hallmark standard. Pt900 — 90.0 percent pure — is widely used in Japan and East Asia. Pt850 — 85.0 percent pure — is the minimum legal platinum fineness in Japan and is common in mass-market goods. Pt999 is occasionally used for specialist work such as wedding bands where maximum purity is sought, though the soft alloy compromises wearability.
The alloying element in fine work is most often ruthenium, iridium, or cobalt at the few-percent level. Each modifies the alloy's hardness, workability, and casting behaviour differently, and bench preferences vary by region and house style.
Setting and wear
Platinum's density and ductility make it the preferred metal for prong settings on important coloured stones and diamonds because the prongs hold tightly and resist deformation under normal wear. The metal develops a characteristic patina over time as the polished surface acquires fine scratches that are not lost from the metal but only displaced — unlike plated white gold, where rhodium wears off the underlying alloy. Patina is preferred by many collectors and can be polished off when a high finish is wanted.
In the trade
Platinum commands a premium over comparable gold work because of higher metal cost and significantly higher fabrication cost. Bench labour in platinum is slower because of the higher melting point and the work-hardening behaviour of the alloy, and casting losses are higher. The premium is concentrated at the high end of the market: bridal, important coloured-stone jewellery, and signed pieces by prestige houses.