How to read gold hallmarks: 375, 585 and 750

The number is a fineness, not a carat

375 does not count anything you can see. It is millesimal fineness: parts of pure gold per thousand parts of alloy. 375 means 375 parts in 1,000, or 37.5 per cent gold by weight, the rest being copper, silver, zinc or palladium to give the metal colour and hardness.

The older carat scale is the same fact expressed out of 24. Nine parts gold in twenty-four is 9 ÷ 24 = 0.375, hence 9 carat and hence 375. The two numbers are always struck for the same thing, and a piece may carry either or both. A gemstone carat is a unit of weight, 0.2 grams, and has nothing to do with this.

The marks and what they convert to

The other metals use the same system. 925 is sterling silver, 92.5 per cent; 958 is Britannia silver; 800 is a continental silver standard common on older European pieces. PT950 or a bare 950 is platinum at 95 per cent, with PT900 and PT850 used for lighter alloys; PD950 and PD500 are palladium.

A full British hallmark has three compulsory parts

Under the Hallmarking Act 1973, a piece assayed in the United Kingdom carries three marks struck by one of four assay offices.

1. The sponsor’s mark. Two or more initials inside a shaped shield, registered by the company that submitted the piece – maker, importer or retailer. It records who put the article on the market, not necessarily who worked at the bench.
2. The fineness mark. The number itself, in a surround whose shape indicates the metal. It may be accompanied by a traditional pictorial mark, which has been optional since 1999: a lion passant for sterling silver, a crown for gold, an orb for platinum.
3. The assay office mark. The town that tested the metal. A leopard’s head is London, an anchor is Birmingham, a rose is Sheffield and a castle is Edinburgh. Glasgow, Chester, Exeter, Newcastle, York and Dublin appear on older pieces.

A date letter – one letter in a shaped shield, the shape and typeface changing each cycle – has been voluntary since 1998. Its presence dates the assay precisely. Its absence dates nothing at all.

Convention marks and import marks

A symbol of a pair of balance scales with the fineness number inside is a Common Control Mark, applied in a country that signed the international hallmarking convention. It is legally recognised in the UK without re-marking, so a Swiss or Scandinavian piece may carry it instead of a British office mark.

British hallmarking is compulsory only above small weight thresholds: one gram for gold, 7.78 grams for silver, half a gram for platinum and one gram for palladium. A fine gold chain, a pair of light studs or a thin stacking band can be sold entirely legally with no mark on it.

American stamps are a different system

The United States has no assay offices and no compulsory hallmarking. What you find is a quality stamp applied by the manufacturer: 10K, 14K, 18K, PLAT, STERLING or 925. Nobody independent has tested the metal.

Federal stamping law requires that a quality mark be accompanied by the registered trademark of the firm standing behind it. So a bare 14K with no maker’s mark anywhere on the piece is a reason to look harder, though it is common on older and imported goods and is not proof of anything by itself.

You will also see 14KP, meaning karat plumb: the alloy is exactly 14 carat rather than shaded slightly under, a distinction that mattered when American tolerances allowed a half-carat shortfall. Continental pieces sometimes carry a bare 18 or 750 with a maker’s lozenge and no town mark.

The marks that mean it is not solid gold

These appear beside or instead of a fineness number, and they change the value of a piece by an order of magnitude.

• GP – gold plated. A few microns of gold electroplated over base metal.
• GF, usually as a fraction such as 1/20 12K GF – gold filled. A bonded layer of carat gold making up at least one twentieth of the article’s weight. Far more durable than plate, still not solid.
• RGP or HGE – rolled gold plate and heavy gold electroplate. Thin coatings, whatever the word heavy suggests.
• Vermeil – sterling silver with a gold layer over it, often stamped only 925.
• EPNS – electroplated nickel silver, which contains no silver at all.

The commonest surprise is a warm yellow piece stamped 925: the body is silver and the colour is a coating. Look for wear at the high points, a cooler grey or pink tone showing through, and a green line where plating has failed at an edge.

When there is no mark, or the mark is wrong

An unmarked piece is not a fake piece. Marks go missing for ordinary reasons: the article is below the legal threshold, it was made in a country with no hallmarking regime, it predates the relevant law, it has been resized and the jeweller cut away the stamped section of the shank, or forty years of wear have simply polished the mark flat.

Marks can also lie. Fineness stamps are struck onto plated goods, and a stamp is only as honest as whoever held the punch. The tests that settle fineness are an assay: XRF at a jeweller, which is quick and non-destructive but reads only the top few microns and can be fooled by a heavy gold-filled layer, or an acid and touchstone test, which cuts through a coating and leaves a small mark on the metal.

Photographing a hallmark so it can be read

Marks are struck shallow and are often under a millimetre across, so the photograph does most of the work.

• Clean the area with a dry cloth. Do not polish it, and do not polish an antique piece at all.
• Light it from the side, not from the front. Raking light throws shadow into the punched depressions and makes them legible; direct flash flattens them to nothing.
• Rest the piece on a dark, non-reflective surface and use your phone’s macro mode, backing off slightly if the lens refuses to focus.
• Take the inside of the band, the clasp tongue, the back of a pendant bail and the earring post separately. Marks are split across components more often than people expect.

Carat transcribes what is struck rather than what it assumes – 750 stays 750, not “18 carat gold” – and then interprets the mark, which is the only order that lets you check its reasoning. How it works sets out the rest, and what your jewellery is worth explains how fineness feeds into a melt figure.