A fineness mark is a numeric stamp applied to precious-metal jewellery and objects to declare the purity of the metal in parts per thousand. Where a traditional carat or lot system expresses purity as a fraction — 18 carat gold being 18 parts gold in 24 — the millesimal fineness system converts that fraction to a three-digit integer: 18-carat gold becomes 750 (75.0% pure gold), sterling silver becomes 925 (92.5% pure silver), and platinum at 95% purity is marked 950. The system is internationally recognised, legally mandated in many jurisdictions, and forms the backbone of consumer protection in the precious-metals trade.

The Millesimal System

The term millesimal derives from the Latin millesimus, meaning "thousandth". By expressing metal content as a proportion of one thousand parts, the system eliminates ambiguity across different national carat conventions. Common fineness marks encountered in the jewellery trade include:

• 999 — fine gold or fine silver (99.9% pure); used for bullion and some high-purity jewellery
• 958 — 23-carat gold, sometimes called Britannia gold
• 750 — 18-carat gold (75.0% gold)
• 585 — 14-carat gold (58.5% gold), dominant in North American and German markets
• 375 — 9-carat gold (37.5% gold), common in the United Kingdom and Australia
• 925 — sterling silver (92.5% silver)
• 800 — continental silver, widely used in older European silverware
• 950 — high-purity platinum, the standard for fine jewellery in most markets
• 850 and 900 — lower-grade platinum alloys

Legal Framework and Assay Offices

In the United Kingdom, fineness marks are applied by one of four statutory Assay Offices — London, Birmingham, Edinburgh, and Sheffield — under the Hallmarking Act 1973. A complete UK hallmark comprises the sponsor's (maker's) mark, the millesimal fineness mark, and the Assay Office mark; a date letter, though no longer compulsory for most articles after 1999, is still widely used. The fineness figure appears within a distinctive shield or cartouche whose shape varies by metal: an elongated octagon for gold, an oval for silver, and a pentagon for platinum, allowing rapid visual identification even without reading the numerals.

Across continental Europe, the Convention on the Control and Marking of Articles of Precious Metals — commonly called the Vienna Convention or the Hallmarking Convention — establishes a common control mark (the scales-and-balance symbol) accepted by signatory states including the United Kingdom, Switzerland, Austria, Sweden, and others. Articles bearing this convention mark alongside a fineness number may be sold across member states without re-assaying, facilitating cross-border trade.

Fineness Marks Versus Hallmarks

The terms are related but not synonymous. A hallmark is the complete set of official stamps applied by an assay authority, of which the fineness mark is one component. In everyday trade usage, however, "hallmark" is often used loosely to mean any official purity stamp, and the fineness number alone is frequently described as a hallmark. Strictly, a piece bearing only a maker's self-declaration of fineness — without independent assay-office verification — carries a fineness stamp rather than a true hallmark. This distinction matters legally: in the UK and many EU countries, selling an article as gold or silver without a proper hallmark is a criminal offence above certain weight thresholds.

Practical Significance in the Trade

For gemmologists, jewellers, and collectors, the fineness mark is the primary documentary evidence of metal content when no laboratory analysis is available. When assessing an antique or estate piece, the mark's style, cartouche shape, and accompanying office symbols can assist in dating and attributing the object. Absence of a fineness mark on a piece purporting to be precious metal warrants independent testing — typically X-ray fluorescence (XRF) spectroscopy or fire assay — before any valuation or resale.

It is also worth noting that fineness marks attest only to the alloy composition at the time of assay; they do not speak to plating, filled constructions, or subsequent repairs in a different metal. A piece stamped 750 may have later solder joints or findings in a lower-carat alloy, a detail that careful examination under magnification will often reveal.

Further Reading

• World Gold Council — Hallmarking overview
• The Assay Office Birmingham — About Hallmarking