The short version: 375 stamped on gold means 9ct, 750 means 18ct, and a genuine UK hallmark is not one stamp but a row of them, applied by an independent Assay Office after testing the metal. If a piece of gold jewellery over one gram is sold in the UK without a hallmark, it is being sold illegally. That row of tiny symbols is the single most reliable thing about any piece of gold you own.

I am Patrycja, founder of THERESHEIS.jewels in Edinburgh. Every piece I sell is independently assayed and hallmarked, and I spend a lot of time explaining these little marks to customers. So here is the whole thing in one place, in plain English.

What does 375 mean on gold?

375 means the metal is 37.5% pure gold, which is 9ct. The number is called a millesimal fineness mark, and it tells you parts of gold per thousand. So 375 is 9ct, 585 is 14ct, 750 is 18ct, and 916 is 22ct.

This is why "9ct gold" and "375 gold" are the same thing. The rest of the alloy is other metals, usually silver, copper and zinc, and they are the reason 9ct is harder wearing than 18ct. More on that trade-off in my piece on 9ct gold, but the fineness number itself is simple: higher number, more pure gold, softer metal, higher price.

What makes a full UK hallmark?

A legal UK hallmark has three compulsory parts, struck or laser marked together, usually somewhere discreet like the inside of a ring or the back of a clasp.

The sponsor's mark comes first. Two or more letters in a shield, identifying who submitted the piece for testing. Then the fineness mark, the number in a shaped surround: 375 for 9ct sits in an oval for gold. Finally the Assay Office mark, showing which of the four UK offices tested the metal: a leopard's head for London, an anchor for Birmingham, a rose for Sheffield, and a castle for Edinburgh.

I will admit a soft spot for that last one. Seeing the three-turreted castle on a piece of jewellery means it was tested a few miles from where I pack your order.

Some pieces also carry a date letter showing the year of hallmarking, and sometimes a traditional mark like a crown for gold. These are optional extras. The three compulsory marks are what the law requires.

Is a hallmark legally required in the UK?

Yes, for gold items over 1 gram. Under the UK Hallmarking Act it is an offence to describe or sell an unhallmarked item as gold above that weight. The testing is done by an independent Assay Office, not by the seller, which is exactly why the mark means something. Nobody can hallmark their own claims.

This is also the quiet difference between UK-hallmarked jewellery and a lot of what sells online. A stamp applied in a factory is a claim. A hallmark is a verdict.

How is a hallmark different from a "gold" stamp?

Look out for stamps that imitate the real thing. "GP" means gold plated. "GF" or "RGP" means gold filled or rolled gold plate. "925" on a gold-coloured piece means it is sterling silver underneath, usually vermeil. "18KGP" is 18ct gold plating, not 18ct gold.

None of these are hallmarks and none of them mean solid gold. If a listing says "gold" and the only mark is one of the above, you now know what you are actually buying. Solid gold jewellery sold in the UK will have the full three-part hallmark, and a seller of solid gold will be glad you asked about it.

How do I check the hallmark on my own jewellery?

You need decent light and, honestly, a magnifying glass or your phone camera zoomed in. Check the inside of rings, the posts or backs of earrings, the clasp or the tag on chains and bracelets.

Find the fineness number first, because it is the easiest to read. Then look for the Assay Office symbol next to it. If you can see a shield with initials, a number in an oval, and one of the four office marks, you are holding tested solid gold. If you find nothing at all on a piece described as gold, or only a plating stamp, treat the description with suspicion.

One honest caveat: very small or very old pieces can have worn or exempt marks, and antique jewellery follows its own rules. For anything inherited and unmarked, a local jeweller or the Assay Office itself can test it properly.

What does this mean when you shop with us?

Every piece at THERESHEIS.jewels is solid 9ct or 18ct gold and independently hallmarked before it reaches you. Nothing plated, nothing vermeil, nothing "gold tone". You will find the marks on your piece, and every order arrives with a certificate alongside the gift box. If you ever struggle to find the hallmark on something you bought from me, write to me at hi@theresheisjewels.com and I will point you to it.

Frequently asked questions

What does 375 mean on my ring? 375 means the metal is 37.5% pure gold, which is 9ct gold. It is a millesimal fineness mark, showing parts of gold per thousand.

What does 750 mean on gold? 750 means 75% pure gold, which is 18ct. It is the standard fineness mark for 18ct gold in the UK and across Europe.

Is it illegal to sell gold without a hallmark in the UK? Yes. Under the UK Hallmarking Act, gold items over 1 gram cannot legally be sold or described as gold without a hallmark from an independent Assay Office.

Which symbols do the UK Assay Offices use? A leopard's head for London, an anchor for Birmingham, a rose for Sheffield, and a three-turreted castle for Edinburgh.

Does a 925 stamp mean gold? No. 925 is the fineness mark for sterling silver. A gold-coloured piece stamped 925 is silver with gold plating, known as vermeil, not solid gold.

THERESHEIS.jewels is an independent fine jewellery brand in Edinburgh. Everything we sell is solid 9ct or 18ct gold, UK hallmarked, and covered by our 2 year warranty.