A 750 stamp on jewellery means the metal is 750 parts per 1,000 gold, which works out to 75.0% fine gold, the same thing as 18 carat. That part’s simple maths. What’s not quite so simple is that a lone 750 stamp, on its own, isn’t the same as a proper UK hallmark. Stick with us and we’ll show you exactly how to tell the difference.
TL;DR:
- A valid UK hallmark combines a sponsor’s mark, fineness mark, and assay office symbol; a lone 750, 18K, or Au750 stamp is only a claim.
- In the UK, gold items weighing more than 1 gram must carry a full hallmark when sold as gold; the exemption applies below that threshold.
- Photograph every mark, compare the assay symbol with reference images, and avoid home acid kits, which can damage the piece; professional testing can check composition.
- For resale, melt value depends on weight and fineness, while craftsmanship, stones, brand, age, and condition shape a piece’s broader market value.
Table of Contents
- What ‘750’ means and why people call it 18 carat
- How 750 and 18ct compare with 375, 585, 916 and 999
- Hallmark parts explained: what to actually look for
- UK hallmarking rules and legal minimums
- How to check if a 750 mark is genuine
- Looking after 18ct gold jewellery day to day
- How we authenticate and service 18ct pieces
- Where the 750 hallmark came from
- 750 gold versus gold-plated: spotting the difference
- How 750 purity affects what a piece is actually worth
- Fake and misleading 750 marks: what to watch for
- Why checking the hallmark matters more than people think
- How we can help with authentication, repairs and valuations
- FAQ
- Sources
What ‘750’ means and why people call it 18 carat
Right, let’s get the maths out of the way, because it’s genuinely not complicated once someone explains it properly (annoyingly, most people never bother). Gold purity gets measured in two systems that basically say the same thing in different languages: millesimal fineness and carats.
Millesimal fineness is just parts per 1,000. So 750 means 750 out of every 1,000 parts are pure gold, and the other 250 parts are other metals. Do the division and you get 75.0% gold, which the Hallmarking Act’s own schedule confirms maps directly onto 18 carat, since carats work on a scale of 24 (as in, 18 out of 24 parts gold).
The remaining quarter of an 18ct piece is alloy, and what goes into that alloy decides the colour you end up wearing:
- Yellow gold typically uses silver and copper in the mix, keeping that warm, traditional tone.
- White gold leans on palladium or nickel to bleach out the yellow.
- Rose gold uses a higher proportion of copper for that pink blush.
Goldsmiths favour 750 for a reason: it’s soft enough to shape into fine detail but hard enough to survive being worn every day, which is why you’ll see it on engagement rings, wedding bands, and the kind of chain you don’t want to bend the first time it catches on a jumper.
How 750 and 18ct compare with 375, 585, 916 and 999
Here’s where people usually get confused, because jewellery shops throw numbers around like they’re obvious. They’re not, until you’ve seen them laid out side by side:
- 375 (9ct): 37.5% gold, the UK’s most common budget carat, tough as old boots but noticeably paler in colour.
- 585 (14ct): 58.5% gold, a popular middle ground in the US and parts of Europe.
- 750 (18ct): 75.0% gold, rich colour, the standard for most fine jewellery and bridal pieces.
- 916 (22ct): 91.6% gold, favoured in South Asian and Middle Eastern jewellery traditions, very soft.
- 999 (24ct): 99.9% gold, essentially pure, too soft for anything that takes daily knocks.
The pattern is straightforward: more gold means a deeper colour and a softer metal, less gold means better scratch resistance but a flatter tone. An 18ct ring will hold up to decades of wear far better than a 24ct one, which would dent if you looked at it wrong. On resale, higher purity generally means a higher melt value gram for gram, though the actual price you’re offered depends on workmanship, stones and condition too, not just the fineness stamp.
Hallmark parts explained: what to actually look for
This is the bit that trips people up, so let’s be precise about it. A genuine UK hallmark isn’t one mark, it’s a cluster of marks, and GOV.UK guidance is clear that three of them are compulsory:
- Sponsor’s mark: identifies the maker or company that submitted the item for testing.
- Fineness mark: the number itself, such as 750, showing the gold content.
- Assay office mark: a symbol showing which of the UK’s assay offices tested and verified the piece.
Beyond those three, you might also spot a date letter, which has been optional since 1999 according to the Assay Office, or historical duty marks on older pieces, which don’t affect validity today. You’ll want to compare the actual symbols against reference images, and a guide to identifying the four UK assay office marks is a handy place to start if you’re squinting at a loupe for the first time.
Here’s the catch, though: a simple ‘750’, ‘18K’, or ‘Au750’ stamped alone, with nothing else around it, is often just a manufacturer’s own purity claim. It might be entirely accurate. It might not be. Either way, it’s not independent verification.
UK hallmarking rules and legal minimums
A complete UK hallmark has three compulsory components: sponsor’s mark, fineness mark (such as ‘750’) and assay office mark.
That’s not us being cautious, that’s the actual legal standard set out in GOV.UK’s hallmarking guidance. Under the Hallmarking Act, any gold item weighing more than the legal minimum weight must carry a full hallmark if it’s going to be sold and described as gold. That threshold exists because hallmarking an item does cost something, and tiny pieces (think delicate chain links) would be disproportionately affected, but once you’re over 1.0 gram, there’s no wiggle room.
What does that protect you from, practically? Quite a lot, actually. A proper hallmark means an independent assay office has tested the metal and confirmed it meets the fineness claimed, not just trusted the seller’s word for it. Sell an item described as gold above that weight threshold without a genuine hallmark, and that’s a breach of UK law, full stop. It’s not a grey area or a nice-to-have.
So when you’re holding a piece with nothing but a ‘750’ stamped into it and no sponsor’s mark, no assay symbol, that’s worth noticing. It doesn’t automatically mean the piece is fake (plenty of older or imported items slip through without full compliance), but it does mean you’re relying on trust rather than verification, and that’s a different thing entirely.
How to check if a 750 mark is genuine
Right, you’ve found a 750 stamp and you want to know if it’s the real deal. Here’s the order we’d actually do this in:
- Find all the marks, not just the 750, using a jeweller’s loupe or a decent phone camera zoomed right in.
- Check the position and clarity of the stamp, as genuine hallmarks are crisp, consistent, and usually tucked inside a band or on a clasp.
- Compare the assay symbol against reference images, since each UK assay office uses a distinct pictorial mark.
- Photograph everything before you do anything else, because a good clear image is useful evidence either way.
- Book a professional inspection if you’re still unsure, rather than guessing.
A word of warning on DIY testing: those home acid test kits and magnet tricks you’ll see recommended online are unreliable at best, and acid tests can actually damage the piece. Professional testing, usually X-ray fluorescence (XRF), is non-destructive and gives you a proper composition reading without putting a mark on the metal, as PreciousMetalPrices’ glossary notes. If you want the full walkthrough, our piece on how to authenticate gold jewellery goes through this in more depth.
Pro Tip: Keep any paperwork, receipts or valuations that came with the piece, as provenance documents speed up authentication enormously.
Looking after 18ct gold jewellery day to day
Eighteen carat wears differently to the cheaper carats, and it’s worth knowing what to expect before you put a ring on and forget about it for a decade. It’s softer than 9ct or 14ct, so a daily-worn ring will pick up fine scratches faster, though it’s also less brittle, meaning it bends rather than snaps under stress.
A soft brush, warm soapy water, and a soft cloth will keep most pieces looking sharp between proper cleans. Watch for:
- A loose stone or a setting that’s started to wobble.
- A shank that feels noticeably thinner than it used to.
- Any dulling that a gentle polish doesn’t shift.
If you’re wearing white gold, bear in mind the bright white finish usually comes from rhodium plating, which wears thin over years and needs reapplying. That replating process can occasionally affect how crisp an existing hallmark looks, which is worth mentioning to whoever’s doing the work.
How we authenticate and service 18ct pieces
When an 18ct piece comes through our doors, whether it’s for valuation, resale or repair, we run a consistent process. We start with a visual hallmark check under magnification, then weigh and measure the piece against what’s expected for that style, then cross-reference any documentation the owner has brought along.
From there, our in-house jewellers carry out a closer inspection, and where there’s genuine doubt, we arrange authorised testing before anything is valued or sold. This is the same rigour we apply to every piece that enters our second-hand jewellery collection, because a hallmark claim and a verified hallmark are two different things, and we only sell the latter.
Services that tend to come up alongside authentication include:
- Valuation for insurance or resale purposes.
- Structural repairs and rhodium replating.
- Pawnbroking against verified pieces.
If you’re bringing something in, a few photos of the marks and any existing paperwork will make the appointment quicker for everyone.
Where the 750 hallmark came from
The millesimal fineness system, the one that gives us 750, 585 and 375, grew out of a need for a universal language around gold purity that didn’t depend on each country’s own carat conventions. Carats themselves trace back centuries, originally tied to the weight of carob seeds used to balance scales, before evolving into the 24-part scale we use today.
Britain’s hallmarking tradition is older still, running back to medieval guild regulation of precious metals, long before anyone wrote ‘750’ on anything. The numeric millesimal marks were adopted more widely across Europe through the 20th century as international trade in jewellery grew and manufacturers needed a purity mark that read the same whether the item ended up in London, Paris or Milan.
The ‘750’ figure itself isn’t arbitrary. It sits exactly at 18 out of 24 parts, chosen because it strikes a workable balance: enough gold to carry real value and colour, enough alloy to give the metal strength. That’s why it became the standard for fine jewellery across so many markets rather than a quirk specific to one country.
Today, a 750 stamp sits alongside the UK’s own hallmark cluster, sponsor’s mark, assay office symbol and fineness number together, continuing a system that’s had centuries to get the balance between tradition and practicality right.
750 gold versus gold-plated: spotting the difference
This distinction matters more than people realise, because gold plating and solid gold can look identical on day one and completely different by year two. Gold plating means a thin layer of gold, sometimes genuinely very thin, has been deposited over a base metal like brass or copper.
The giveaway is usually the stamp itself. A genuine 750 or 18ct piece carries that fineness mark as part of a proper hallmark cluster. Plated items are typically marked ‘GP’ (gold plated), ‘GEP’ (gold electroplate), or sometimes carry no metal content mark at all, just a brand name.
Wear tells the real story over time. Solid 750 gold keeps its colour and composition for life, a scratch just reveals more of the same gold underneath. Plated jewellery will eventually wear through at contact points, the inside of a ring, the back of a pendant, revealing the base metal beneath, often as a dulling or a different coloured patch.
If you’re ever unsure which you’re holding, the weight can be a clue (solid gold feels notably heavier for its size than most base-metal alternatives), but the only certain answer comes from checking the hallmark properly or having the piece tested. Our guide on spotting fake gold jewellery covers more of the warning signs worth knowing before you buy.
How 750 purity affects what a piece is actually worth
Purity is one of the biggest single factors in what 18ct jewellery is worth, but it’s not the only one, and treating it as the whole story will lead you astray. The gold content sets a baseline: gram for gram, 750 gold carries more intrinsic metal value than 375 or 585 pieces, simply because there’s more actual gold in it.
From there, though, value splits into two separate conversations. Melt value is what the raw gold content is worth if the piece were broken down, and that’s driven almost entirely by weight and fineness. Resale or retail value is a different animal entirely, shaped by craftsmanship, brand, age, stone quality and condition, which is why a beautifully made 18ct piece can be worth considerably more than its melt value alone would suggest, and why a damaged or heavily worn one might sell closer to scrap price regardless of its purity stamp.
This is exactly why authentication matters before any valuation conversation happens. A verified hallmark confirms the purity claim is accurate, which protects both the buyer’s expectations and the seller’s asking price. Guessing at purity, or trusting an unverified stamp, means either side of a transaction could be working from the wrong number entirely.
Fake and misleading 750 marks: what to watch for
Not every 750 stamp tells the truth, and it’s worth knowing the common ways people get caught out. The most basic trick is simply stamping a base-metal item with ‘750’ or ‘18K’ and hoping nobody checks further, relying on the fact that most buyers never look past the number itself.
More sophisticated versions involve genuine gold-plated items marked with a fineness number that only applies to the plating, not the item as a whole, which is misleading even if technically there’s some gold present somewhere on the piece. Then there’s the issue of worn or deliberately obscured marks, where a stamp has been filed down or placed somewhere awkward to photograph, making it harder for a buyer to check properly before purchase.
The protections we’ve already covered are exactly how you catch these: look for the full hallmark cluster, not just the number, because a sponsor’s mark and assay office symbol are far harder to fake convincingly and far easier for a professional to verify than a lone digit sequence. Be especially cautious with private sales, online marketplaces and auction lots where there’s no professional standing behind the description. If a seller can’t produce paperwork, won’t let you inspect the marks closely, or the price seems unusually low for the stated purity, treat that as a reason to get independent verification before you commit to anything.
Why checking the hallmark matters more than people think
We’ve lost count of how many pieces come in with a confident ‘18ct’ label and nothing to back it up beyond the seller’s word, usually because nobody along the way thought to look for the assay mark sitting right next to the fineness number. It’s an easy thing to miss if you’ve never been told to look for it.
If you’re buying, inheriting, or selling, do yourself a favour: photograph the marks properly and get a professional opinion when anything looks uncertain. It costs you nothing to be sure, and quite a lot to find out later that you weren’t.
— James
How we can help with authentication, repairs and valuations
If you’ve got a piece with a 750 stamp and you’re after a straight answer rather than a guessing game, that’s precisely where we come in. We check hallmarks properly, weigh and inspect every piece in-house, and we’ll tell you honestly what we find rather than what’s convenient to say.
Beyond authentication, we handle jewellery repairs including rhodium replating and structural work, offer valuations through our pawnbroking and Cash 4 Gold service if you’re considering selling, and stock a second-hand jewellery collection where every 18ct piece has already been through our full inspection and hallmarking process before it ever reaches a shelf.
Bring a piece in for a hallmark check, or browse our certified second-hand collection if you’d rather buy with the verification already done for you.
FAQ
What is the code number for 18K gold?
The code number for 18K gold is 750, representing 750 parts per 1,000, or 75.0% pure gold. This figure comes directly from the Hallmarking Act’s statutory table mapping carats to millesimal fineness.
What does 925 mean on jewellery?
A 925 stamp indicates sterling silver, meaning 925 parts per 1,000 (92.5%) are pure silver, with the remainder typically copper for added strength. It follows the same millesimal logic as gold’s 750 mark, just applied to a different precious metal.
Can fake gold be stamped 750?
Yes, a base metal or gold-plated item can be stamped ‘750’ without any independent verification behind it, since a lone number isn’t a legal hallmark on its own. GOV.UK guidance advises checking for the sponsor’s mark and assay office symbol alongside the fineness number before trusting the claim.
Is 750 gold equal to 18K?
Yes, 750 and 18K describe the exact same gold purity, just using two different measurement systems, millesimal fineness and carats respectively.
