Gold Hallmarks and Stamps Explained
A complete decoder for the numbers and letters stamped on gold, including the marks that mean it is not gold at all.
Updated July 29, 2026
The two notations for the same thing
Gold purity gets written two ways. Karat expresses it in 24ths: 18K means 18 parts gold to 6 parts alloy. Millesimal fineness expresses it in thousandths: 750 means 750 parts gold per 1,000. They are the same statement, and 18K and 750 are interchangeable. Most of the world stamps fineness. The US is the main holdout that stamps karat, usually as "14K" or "14KT". If you find both on one piece, they should agree, and a mismatch is a reason for suspicion. To convert either direction: karat ÷ 24 × 1000 = fineness, and fineness ÷ 1000 × 24 = karat. So 22K is 22 ÷ 24 × 1000 = 917, stamped as 916 by convention.
The fineness numbers you will actually see
999 or 999.9 - 24K, 99.9% or 99.99% gold. Bullion bars and coins, and jewelry in parts of East and Southeast Asia. Too soft for most Western jewelry use. 990 - 23.8K, sometimes seen on Chinese jewelry as "chuk kam" high-purity work. 958 - 23K. Common in Thai jewelry. 916 or 917 - 22K, 91.67% gold. The South Asian jewelry standard, and the alloy of the Krugerrand and American Gold Eagle. 875 - 21K, 87.5% gold. The Gulf states standard, particularly Saudi Arabia, the UAE and Kuwait. 833 - 20K, seen on some older Middle Eastern and Asian pieces. 750 - 18K, 75% gold. The European and luxury standard: fine jewelry, watch cases, designer work. Best compromise between value and durability. 585 - 14K, 58.33% gold. The dominant US standard and the most common karat in American scrap lots. 583 - also 14K. An older US convention reflecting the legal half-karat tolerance on unplumbed gold. Not a defect. 417 - 10K, 41.67% gold. The US legal minimum that may be sold as "gold". Budget jewelry, class rings, mass-market chain. 375 - 9K, 37.5% gold. UK, Ireland and Australian standard. Cannot legally be sold as gold in the US. 333 - 8K, 33.3% gold. German and some other European jewelry. Legal in Germany, not in the US or UK.
Marks that mean it is not solid gold
GP or GEP - gold plated or gold electroplated. A microscopic layer, typically 0.5 to 2.5 microns, over brass, copper or silver. Essentially zero melt value. HGE - heavy gold electroplate. Thicker than standard plating but still measured in microns. No meaningful melt value. RGP - rolled gold plate. A gold sheet mechanically bonded to base metal. Thicker than electroplate, still not refinable in most buyers' eyes. GF, and fractional forms like 1/20 12K GF or 1/10 14K GF - gold filled. A substantial bonded layer where the fraction gives the gold proportion by weight: 1/20 means 5% of the total weight is 12K gold. This is genuinely refinable, and on a heavy piece the recoverable gold is not nothing, but most buyers decline it because the processing cost per gram recovered is high. Vermeil - at least 2.5 microns of at least 10K gold over sterling silver. Sold as fine jewelry, and the silver underneath has its own scrap value, but it is not gold scrap. A trap worth naming: "18KGP" and "14K GP" look like karat marks with a suffix. The GP governs. The 18K describes the plating alloy, not the item.
Other letters and symbols on the same stamp
A "P" after the karat, as in 14KP or "14K plumb", means plumb gold - guaranteed to be a full 14K with no half-karat tolerance applied. Maker's marks and registered responsibility marks are letter or symbol stamps identifying who made or sponsored the piece. These are worth researching - a recognised mark can multiply the value of an otherwise ordinary item, and it is the fastest way to discover you are holding designer work. Assay office marks and date letters appear in countries with compulsory hallmarking. UK marks in particular are a full record: sponsor's mark, fineness in a shaped shield, an assay office town mark, and a date letter, which together date a piece to a specific year and city. "CW", "SG", or similar next to gemstones usually indicates a stone type or a certified weight, not a gold mark. Numbers like "0.50ctw" describe carat weight of stones, which has nothing to do with karat purity of gold - the two words are unrelated despite sounding identical.
Regional hallmarking systems
United Kingdom. Compulsory hallmarking for items over 1 gram, administered by four remaining assay offices - London, Birmingham, Sheffield and Edinburgh. The full mark set gives sponsor, fineness, office and year, so British pieces are the easiest in the world to date and attribute. India. BIS hallmarking is now mandatory for gold jewelry, and marked pieces carry the BIS logo, the fineness, and a six-digit alphanumeric HUID that can be verified against the BIS register. Continental Europe. The Vienna Convention common control mark is recognised across signatory states, alongside national systems. France uses distinctive symbol punches - an eagle head historically indicated 18K. United States. There is no compulsory assay system. The National Gold and Silver Stamping Act requires that any mark applied be accurate, and stamping itself is voluntary. This means an American piece can be genuine and unmarked, or marked and counterfeit, with no official register to check against.
Why a stamp is not proof
Hallmarks are stamped with punches, and punches can be made by anyone. Counterfeit jewelry is routinely marked with correct-looking fineness stamps, and in unregulated markets the mark carries no legal weight at all. Absence of a mark is also weak evidence. Stamps wear off high-friction surfaces, get polished away during repair, are omitted on handmade and antique pieces, and are sometimes on a component that has been replaced. The practical approach is to treat the hallmark as the hypothesis and then verify it. A density measurement takes two minutes with a scale and a cup of water and will catch most brass fakes. XRF gives a definitive alloy breakdown in seconds and most buyers will run one in front of you. A stamp plus a matching density reading is a reasonable basis for a decision; a stamp alone is not.
Key points
Millesimal fineness is parts per thousand: 750 = 75% gold = 18K. 585 = 14K, 417 = 10K, 916 = 22K, 375 = 9K, 999 = 24K. GP, GEP, HGE, RGP, GF and vermeil all mean surface gold over base metal. A stamp is a claim, not a guarantee - forged marks are common.