Short answer: the number is the metal's purity in parts per thousand. AU750 (also written 750 or 18K) is 75% gold, PT950 is 95% platinum, and S925 (or 925, Sterling) is 92.5% silver. Letters such as GP, GF or HGE after a number mean the gold is only a surface layer.
Turn a ring over and look inside the band, or check the clasp of a necklace or the post of an earring: most fine jewelry carries a tiny stamp. It is the maker's declaration of what the metal is. The code looks cryptic, but it follows one simple rule.
The rule: parts per thousand
Jewelry stamps express purity out of 1,000. A piece stamped 750 is 750 parts pure gold in every 1,000, which is 75%. The rest is other metals added to make it harder and to set its colour. The letters in front name the metal: AU for gold (from the Latin aurum), PT for platinum and S for silver.
Karats are an older way of saying the same thing for gold, out of 24 instead of 1,000. 18 parts out of 24 is 75%, so 18K and 750 describe the same gold.
Gold stamps
| Stamp | Also written | Gold content | Notes |
|---|---|---|---|
| 999 | 24K, AU999 | 99.9% | Pure gold. Too soft for settings that hold a diamond. |
| 916 | 22K, AU916 | 91.6% | Rich yellow; still soft for everyday rings. |
| 750 | 18K, AU750 | 75% | The usual fine-jewelry standard: strong enough for prongs, rich in colour. |
| 585 | 14K | 58.5% | Harder and paler than 18K; common in the US. |
| 375 | 9K | 37.5% | Common in the UK; the least gold of the solid alloys. |
Colour is not part of the number. Yellow, rose and white 18K gold are all 75% gold; what changes is the other 25%. Rose gold carries more copper, and white gold is alloyed with white metals and is usually finished with a bright rhodium layer. You will sometimes see a letter added for colour, such as 18KW for white or 18KR for rose. Our guide to yellow, rose or white gold covers how each colour wears.
Platinum stamps
| Stamp | Also written | Platinum content | Notes |
|---|---|---|---|
| PT950 | 950, 950 PLAT, PLAT | 95% | The standard for fine platinum jewelry. |
| PT900 | 900 PLAT | 90% | Common in Japan; a little less platinum. |
| PT999 | 999 | 99.9% | Very pure and soft; used mainly for simple pieces. |
Platinum is naturally white, so there is no plating to wear away. Instead it develops a soft satin sheen with wear, called a patina, which a jeweller can polish back to a mirror finish. See platinum patina explained.
Silver stamps
| Stamp | Also written | Silver content | Notes |
|---|---|---|---|
| S925 | 925, Sterling, STER | 92.5% | Sterling silver: the standard for silver jewelry. |
| S990 / 999 | Fine silver | 99–99.9% | Very soft; rarely used to hold stones. |
The other 7.5% of sterling silver is usually copper, which is why silver darkens over time as it reacts with sulphur in the air. It polishes back; sterling silver care shows how.
Marks that mean plated, not solid
These letters change the meaning of a number completely. When you see them, the precious metal is a layer over something else:
| Mark | Means |
|---|---|
| GP, GEP | Gold plated / gold electroplated: a thin gold layer over a base metal |
| HGE | Heavy gold electroplate: a thicker plated layer, still not solid gold |
| GF, RGP | Gold filled / rolled gold plate: a bonded gold layer over a base metal |
| 18K GP | 18K-coloured plating, not 18K gold |
| 925 on a gold-coloured piece | Gold-plated sterling silver (often called vermeil) |
What a stamp does not tell you
- It says nothing about the stones. A metal stamp describes the metal only.
- It is a declaration, not a test. Some countries, such as the UK, require pieces above small weights to be independently assayed and hallmarked; elsewhere the stamp is the maker's word. A jeweller can confirm the metal with an acid test or an X-ray fluorescence reading.
- Small pieces may have no stamp at all simply because there is no room: fine chains, tiny studs and thin bands.
Which stamp suits a diamond?
For a stone worn every day, the metal holding it matters as much as the stone. 18K gold (AU750) and PT950 platinum both hold prongs securely for years; sterling silver is softer and suits pieces worn less often, or designs where the setting takes fewer knocks. Our side-by-side guide, 18K gold, platinum or sterling silver, and the head-to-head PT950 vs S925 go further.
Every Aetheris design is made to order in 18K gold (yellow, rose or white), PT950 platinum or S925 sterling silver — browse lab-grown diamond jewelry.
Frequently asked questions
Is AU750 real gold?
Yes. AU750 is 75% gold, the same as 18K. It is solid gold, not plating, unless a plating mark such as GP follows it.
Is PT950 better than 18K gold?
Neither is better in every way. PT950 is denser, naturally white and never needs re-plating; 18K gold comes in yellow, rose and white and keeps a bright polish longer. Both are strong enough to hold a diamond for everyday wear.
Does S925 tarnish?
Yes. The copper in sterling silver reacts with sulphur in the air and darkens the surface over time. It cleans off with a silver cloth and does not damage the metal.
What does 925 mean on gold-coloured jewelry?
It means the piece is sterling silver with a gold-coloured plating on the surface, not solid gold.