Carat

Carat, as applied to gold, is used to mean simply 1/24th part by weight of the substance. Thus 18 carat gold signifies that the article consists of 18/24ths, or 75 per cent. pure gold. The gold used in our current coinage consists of 91.66 pure gold, or 22 carats. The carat as used for weighing precious stones differs in different countries, but for diamonds, a convention of the diamond merchants of London, Paris, and Amsterdam agreed in 1877 that the carat, equivalent to 4 diamond grains, should be 205 milligrams, and should be divided by 4ths, 8ths, 16ths. and so forth. The tiny platinum weights used by diamond merchants are some of them hardly more than a film. The word, of Greek origin through Arabic, originally denoted a kind of seed.