Fineness

Fineness is a proportion, not a quality. The UK government's hallmarking guidance describes the second compulsory part of a UK hallmark as the metal and fineness mark, giving "the millesimal number (parts per thousand) indicating purity". The standards it recognises are, for gold, 375, 585, 750, 916.6, 990 and 999; for silver, 800, 925, 958 and 999; for platinum, 850, 900, 950 and 999; and for palladium, 500, 950 and 999.

Karat is the same proportion counted out of 24 instead of 1,000. India's Bureau of Indian Standards, working to IS 1417:2016, permits six grades for gold jewellery and publishes both figures together: 14K at 585 parts per thousand, 18K at 750, 20K at 833, 22K at 916, 23K at 958 and 24KS at 995. A case marked 750 and one marked 18K make the same statement.

There is an international standard: ISO 9202:2026, Jewellery and precious metals — Fineness of precious metal alloys, edition 4, published May 2026, which "specifies a range of fineness of precious metal alloys recommended for use in the field of jewellery". What this Codex does not do: quote its table of grades. Only ISO 9202's abstract is public, so only that has been read; the millesimal figures above come from the UK hallmarking guidance and the caratage pairings from the Bureau of Indian Standards.

One word does double duty. A gemstone's carat is a mass, not a proportion. NIST writes that "The carat (c) is another common measurement unit used in jewelry sales to measure gemstones. The carat had its origin in the carob seed, which later standardized a 1/444 ounce and then 0.2 gram (or 200 mg)." So 0.2 g of diamond and the 18 parts in 24 that the Bureau of Indian Standards records as 18K gold are different quantities that share a name.