PURITY
Carats count in 24ths
Pure gold is 24 carat. Pure gold is too soft for many uses, so it is mixed with other metals: 18 carat is 75% gold, 9 carat is 37.5%.
Pick a caratGold is element 79, and it barely reacts with anything.
That is why old gold still gleams, why it ends up in phone connectors as well as rings, and why the number stamped inside a ring tells you so much. Here is gold the metal, from carats to hallmarks.
Updated
Read a caratTHE SHORT ANSWER
Soft, heavy,
and hard to spoil.
Three things make gold unusual.
Each one shows up in everyday objects.
CHEMISTRY
It does not corrode
Gold is chemically unreactive, which is why it is used for electrical contacts. It does dissolve in aqua regia, a mix of strong acids.
Why it is yellow
SPREAD
It goes a long way
One gram can be beaten into a sheet of about a square metre, just 230 atoms thick, or drawn into about 165 metres of wire.
See the numbersREAD THE NUMBER
How much of it
is gold?
Pick a carat to see how much gold it contains and what a UK hallmark would show. The bar shows the share of gold.
Carat figures from The Royal Mint; hallmark standards from the UK Assay Offices' Hallmarking Guidance Notes (2022).
TWO NUMBERS WORTH KNOWING
All the gold ever mined
is surprisingly small.
One figure for the world's whole stock,
one for how far a single gram stretches.
≈ 222,600 tonnes
Gold mined throughout history
The World Gold Council puts the total at around 222,600 tonnes by mid-2026, about two-thirds of it mined since 1950. Together it would form a cube about 22.6 metres a side.
EVER MINED · WORLD GOLD COUNCIL
230 atoms
Thickness of 1 g beaten to 1 m²
The Royal Society of Chemistry says a gram of gold can be beaten into a square metre sheet just 230 atoms thick. Gold is also dense: 19.3 grams per cubic centimetre.
GOLD LEAF · ROYAL SOCIETY OF CHEMISTRY
The World Gold Council estimates that about 45% of all above-ground gold, roughly 99,700 tonnes, is held as jewellery, and about 18% by central banks.
READING A UK HALLMARK
Three marks
tell the story.
In the UK it is an offence to describe an item as gold unless it is hallmarked, with some exemptions. A full hallmark has three compulsory marks.
Find the sponsor's mark
At least two letters inside a shaped surround. It identifies the maker or sponsor responsible for the item.
Read the fineness mark
A number such as 375, 585, 750 or 916 gives the gold content in parts per thousand.
Spot the Assay Office mark
This shows which of the UK's four Assay Offices tested it: London, Birmingham, Sheffield or Edinburgh.
Remember the 1 gram rule
Gold items under 1.0 gram are exempt from compulsory hallmarking, so a tiny chain may have no mark.
WHY GOLD IS YELLOW
Einstein in
your earrings.
Gold's nucleus carries 79 positive charges, so its electrons move at speeds close to the speed of light. Chemists at Heidelberg University explain that this relativistic effect is stronger in gold than in any other stable element.
The effect shows up in the outermost electron shell, which controls colour. That is part of why gold looks yellow while its neighbour silver looks colourless, and why gold behaves more like copper than silver in some of its bonds.
Questions
Questions people ask
What does 24 carat mean?
Carat is a 24-part ratio, so 24 carat means all 24 parts are gold. The Royal Mint gives 22 carat as 91.7% gold, 18 carat as 75% and 9 carat as 37.5%.
Why is most jewellery not pure gold?
Pure gold is too soft for many uses, so it is alloyed with other metals to make it more durable. The Royal Society of Chemistry says 18 and 9 carat are common for this reason.
Does gold tarnish?
Gold itself is chemically unreactive and does not corrode, according to the Royal Society of Chemistry. It can be dissolved by aqua regia, a mixture of strong acids.
Does all gold jewellery in the UK have to be hallmarked?
Items described as gold must be hallmarked under the Hallmarking Act 1973, but gold items under 1.0 gram are exempt. There are also exemptions for some older items.
How much gold has ever been mined?
Around 222,600 tonnes, according to the World Gold Council's figures for mid-2026. About two-thirds of it has been mined since 1950.
Sources
Where the facts come from
- Royal Society of Chemistry periodic table: Gold
- Assay Offices of Great Britain: Hallmarking Guidance Notes (2022), via GOV.UK
- The Royal Mint: Metal fineness explained
- World Gold Council: How much gold has been mined?
- Heidelberg University: Einstein's theory of relativity explains fundamental properties of gold
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