Gold Carat Guide: What Gold Purity Marks Mean

Written by Ps Its Vintage. Last updated 28 April 2026.

Gold marks can look confusing at first. 9ct, 18ct, 375, 585 and 750 are all telling you something about gold purity, but they are not all the same kind of mark.

This guide explains what gold carat means, how it relates to numbers like 375 and 750, why jewellery is usually made as an alloy rather than pure 24ct, which carats you may come across on older and newer pieces, and how historic UK standards such as 12ct and 15ct fit into the story. If you want a broader introduction to official marks, see How to Read Hallmarks in Vintage and Secondhand Jewellery. If you are also wondering why some genuine pieces do not carry a full hallmark, see Why Some Gold Jewellery Has No Hallmark.

Table of Contents

• What Carat Means in Gold
• Carat vs Fineness vs Hallmark
• What Gold Numbers and Carats Mean
• Why Jewellery Is Not Usually 24ct
• What Gold Alloys Are Generally Made Of
• Which Gold Carats You May See
• Old UK Gold Standards and Key Dates
• UK, Ireland and Other Markets
• How to Read Old Gold Marks More Clearly
• FAQs
• You Might Also Like
• Final Thoughts
• About Ps Its Vintage
• References

What Carat Means in Gold

When carat is used for gold, it means purity. The system works in 24 parts. So if a piece is 24ct, it means all 24 parts are gold. If it is 18ct, it means 18 parts are gold and 6 parts are other metals. If it is 9ct, it means 9 parts are gold and 15 parts are other metals.

That is why higher carat gold contains more pure gold, while lower carat gold contains a larger proportion of alloying metals. The carat number is not about weight or size when talking about gold. It is about how much actual gold is in the mix.

In the UK, the spelling carat is used both for gold purity and for gemstone weight, which is one reason people mix the terms up. In gold, carat means purity. In gemstones, carat means weight.

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Carat vs Fineness vs Hallmark

These three terms often get lumped together, but they do slightly different jobs.

Carat tells you the gold purity in 24 parts.
Fineness tells you the gold purity in parts per thousand.
Hallmark is the official mark showing the item has been independently tested and marked to a legal standard.

So, 9ct and 375 are two ways of expressing roughly the same gold purity. A full hallmark is something different. It is not just a purity number on its own. In the UK, a full hallmark normally includes a sponsor’s mark, a fineness mark, and an assay office mark. A simple 375, 750 or 9ct stamp can still be useful, but it is not the same as a complete official hallmark.

If you want a fuller guide to sponsor’s marks, assay office symbols and date letters, see How to Read Hallmarks in Vintage and Secondhand Jewellery. If you want to understand why some genuine older pieces still have no visible hallmark, see Why Some Gold Jewellery Has No Hallmark.

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What Gold Numbers and Carats Mean

The numbers used in gold marks are fineness numbers. They tell you how many parts out of 1,000 are gold. The table below shows the most useful conversions for buyers in the UK and Ireland.

You may also see 990 in official hallmark standards. That is a recognised fineness standard in both the UK and Ireland, but it is not usually the everyday way buyers describe gold jewellery by carat.

Once you know that these numbers are simply parts per thousand, the marks start to feel much less mysterious. They are just a decimal version of the older carat system.

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Why Jewellery Is Not Usually 24ct

Pure gold is beautiful, but it is also soft. That softness is one of the main reasons most jewellery is not made in pure 24ct gold. For everyday wear, jewellers usually need a metal that can better cope with knocks, friction, shaping, clasps, settings and long-term wear.

That is where alloying comes in. Mixing gold with other metals changes the way it behaves. It can improve strength, durability and workability, and it can also affect colour. This is why much everyday jewellery is made in 9ct, 14ct or 18ct rather than pure 24ct.

This does not make lower carat gold fake. It simply means the balance between pure gold and the supporting alloy is different.

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What Gold Alloys Are Generally Made Of

Gold alloys are usually made with a mix of metals such as copper, silver and zinc. The exact recipe can vary depending on the maker, the period, the colour wanted, and the way the piece needs to perform.

Yellow gold often uses a balance of gold with silver and copper.
Rose gold gets its warmer tone from a higher copper content.
White gold is made by mixing gold with white metals such as palladium, silver or nickel, and many white gold pieces are also rhodium plated to create a brighter white finish.

There is no single recipe that applies to every piece, which is why two items of the same carat can look slightly different in colour.

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Which Gold Carats You May See

If you buy secondhand, vintage or antique jewellery, you may come across more than just 9ct and 18ct. Some are current UK standards, some are older UK standards, and some are more common in other markets.

For British jewellery, 9ct, 14ct, 18ct, 22ct and very high purity standards such as 990 and 999 are the recognised modern hallmark standards. Older British pieces may also show 12ct or 15ct. In Ireland, you may also come across 417, 833 and 916 under the current Irish standards.

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Old UK Gold Standards and Key Dates

Older British gold standards make much more sense once you place them in a timeline.

• 1854: 9ct, 12ct and 15ct gold standards were introduced in the UK.
• 1855: gold wedding rings were made liable for hallmarking for the first time.
• 1932: 12ct and 15ct were discontinued, and 14ct was introduced.
• Modern UK hallmarking: official UK hallmarks use three-figure fineness marks such as 375, 585 and 750 rather than carat numbers as part of the hallmark itself.

So if you find an older piece marked 12ct or 15ct, that is not automatically odd. It can fit perfectly well with genuine period jewellery. It is part of the historical development of British gold standards.

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UK, Ireland and Other Markets

Gold standards are not identical everywhere, which is one reason imported jewellery can confuse buyers in both the UK and Ireland.

In the UK, modern gold hallmark standards are 375, 585, 750, 916.6, 990 and 999. In Ireland, gold standards include 375, 417, 585, 750, 833, 916, 990 and 999. That is why a piece sold in Ireland or the UK can still show marks that look unfamiliar at first glance, especially if it was originally made for another market or hallmarked under a different system.

That does not automatically make the piece better or worse. It simply means the local legal standard and trade habits may be different from the British and Irish marks many buyers are most used to seeing.

If you want to look more closely at Irish assay marks and how Dublin hallmarks differ from UK marks, see Understanding Dublin Hallmarks in Irish Jewellery.

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How to Read Old Gold Marks More Clearly

If you are checking an older piece, it helps to separate three questions.

First: what purity is being suggested, such as 9ct, 15ct, 375 or 750?
Second: is it a simple stamp, or a full official hallmark?
Third: does the rest of the piece make sense for the period, style and construction?

A 375 or 750 mark can be useful. A full hallmark gives stronger evidence. A 12ct or 15ct mark can fit older British jewellery. A 333, 417 or 833 mark may point to an imported, Irish or non-UK market piece. Once you stop expecting every gold item to follow one single pattern, the marks start to tell a much clearer story.

If you want help reading the rest of a hallmark, including sponsor marks and assay office symbols, this guide may help: How to Read Hallmarks in Vintage and Secondhand Jewellery.

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FAQs

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You Might Also Like

• How to Read Hallmarks in Vintage and Secondhand Jewellery
• Why Some Gold Jewellery Has No Hallmark
• Understanding Dublin Hallmarks in Irish Jewellery

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Final Thoughts

Gold marks make much more sense once you separate purity, fineness and hallmarking. 9ct and 375 are not random numbers. They are simply two ways of describing the same gold content. The same idea applies to 10ct and 417, 14ct and 585, 18ct and 750, and older standards such as 12ct and 15ct.

Once you know that jewellery is usually alloyed for strength, colour and everyday wear, older and newer gold marks start to feel far less confusing. If you would like to explore more preloved, vintage and antique gold jewellery, you can browse the collections at Ps Its Vintage.

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About Ps Its Vintage

Ps Its Vintage is independently run and specialises in preloved, vintage and antique jewellery. Every piece is chosen for its character, quality and symbolism, with a focus on hallmarks, makers and meaningful design. Whether you’re comparing gold carats, checking purity marks, or trying to understand older gold standards, this guide is here to help you choose with confidence.

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References

• GOV.UK – Hallmarking: practical guidance summary
• GOV.UK – Hallmarking guidance notes
• legislation.gov.uk – Hallmarking Act 1973 Schedule 2
• Assay Office London – What Is a Hallmark?
• Assay Office London – History of Hallmarking
• Birmingham Assay Office – Our Story
• Dublin Assay Office – Legislation
• World Gold Council – Gold Jewellery: Colour, Carat & Purity

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