The Pt950 stamp is the hallmark or maker's mark certifying that a piece of jewellery is fabricated in 950-fineness platinum, an alloy comprising 95.0 per cent platinum and 5.0 per cent secondary metals. It is the dominant platinum standard across the United States, the United Kingdom, and most other Western markets, and the form a buyer is most likely to encounter on a contemporary platinum ring, chain, or pendant.

Forms of the mark

The mark appears in several legal variants. The simplest is 950, occasionally seen alone in jurisdictions where the metal is determinable from accompanying maker's marks. More explicit forms include Pt950, 950 Pt, 950 Plat, and the older spell-out 950 PLATINUM. In the United Kingdom the mark sits alongside an assay-office hallmark and a fineness number in a separate cartouche under the Hallmarking Act 1973. Continental European pieces will sometimes carry the parts-per-thousand figure in a lozenge-shaped cartouche specific to platinum.

Where to look

Stamps are placed on inconspicuous surfaces sized for legibility under loupe magnification. On rings the inside of the shank is conventional; on pendants the bail or back; on necklaces and bracelets the clasp tongue or the cartouche soldered to the spring-ring. On earrings the stamp is usually on the post or the back of the ear-wire. A stamp that does not appear in any of these standard locations is grounds for questioning the metal, though hand-fabricated and bespoke pieces sometimes carry the mark on a less typical surface chosen by the maker.

The 5 per cent

The non-platinum component is most often ruthenium, iridium, cobalt, or copper, chosen for working properties. Ruthenium hardens the alloy and gives a slightly brighter polish; iridium offers excellent castability; cobalt allows the alloy to take a stronger casting and a different polish character; copper increases workability for hand fabrication. Sophisticated buyers will sometimes ask which secondary metal a particular workshop uses, since this affects both setting characteristics and long-term scratch behaviour.

In the trade

For Skyjems, the Pt950 stamp is the everyday platinum standard for engagement and wedding rings, where its hardness and resistance to wear suit a piece intended for daily use. The stamp does not by itself guarantee fineness; it is a representation by the maker that the assay-office or refinery testing has established the platinum content. Pieces sold without a fineness stamp can still be platinum, but a customer asking for certainty should ask for an assay or for the maker's documentation.

Further reading

• GIA — platinum reference
• AGTA — coloured stone and metals trade standards
• International Gem Society — platinum alloys overview