Why "gold" does not mean the same fineness everywhere
Every country that regulates gold jewellery has picked its own set of recognised purities, and the reasons trace back to different histories rather than any technical disagreement about what gold is. Britain's Hallmarking Act 1973 recognises 375, 585, 750, 916, 990 and 999 — and, crucially, requires an independent assay office to test and strike the mark itself, rather than trusting the maker's own stamp. Anything below 375 (9 karat) simply may not be described as "gold" in the UK at all; it is a criminal offence to apply a false description or a counterfeit hallmark.
The United States runs on a completely different model. There is no government assay office and, since the FTC's 2018 revision of its Jewelry Guides, no requirement to mark an item at all. What federal law under the National Gold and Silver Stamping Act of 1906 does set is a floor: if a karat mark is used, 10 karat (417 fine) is the lowest that may legally be sold as "gold" — historically with a tolerance of half a karat without solder, a full karat with solder. Below that floor, an item must be described by its actual fineness rather than called gold.
India, Italy and the Gulf: three more models
India took a different path again. Since June 2021, the BIS (Hallmarking of Gold Jewellery and Gold Artefacts) Order has made hallmarking mandatory nationwide, and every piece now carries a Hallmark Unique Identification Number — a traceable code linking the item back to the specific assaying centre that tested it — alongside the purity mark. 916 fine (22 karat) is the everyday Indian jewellery standard, with 585 (14K) and 750 (18K) sold mainly in lighter, diamond-set pieces.
Italy is stricter still on tolerance — Italian precious-metals law allows a deviation of only one part per thousand between the stamped titolo and the actual content — but almost never uses a karat stamp at all. Instead, Italian pieces carry the millesimal fineness (750, 585 or 375) struck beside a lozenge-shaped mark identifying the responsible manufacturer, a system that has been in continuous use since well before EU harmonisation attempts began. In the UAE and the wider Gulf, regulators such as Dubai Municipality check that a stamped karat is genuine, but they do not set a minimum karat that may be sold at all — what looks like a standard there is really a market convention: 21 karat dominates everyday Gulf jewellery, 22 karat is more common in Indian-style pieces sold in the same souks, and 18 karat is stocked mainly for buyers used to the Western standard.
What this actually matters for
The practical use of knowing a jurisdiction's standard is reading a mark correctly rather than assuming your home country's convention applies. A ring bought in Dubai stamped 21K and a ring bought in London stamped 750 are both genuine gold under their respective law — one is not a lesser or more dubious standard than the other, they are simply different markets. What genuinely differs, and matters more than the number itself, is who stands behind the mark: a UK hallmark means an assay office independently tested the metal, while a US karat stamp means only that the manufacturer applied it and is legally answerable if it turns out to be false. Both are legitimate systems; they simply put the burden of proof in different places.