Carat (mass) Despite its being a non-metric unit, a number of metric countries have used this unit for its limited range of application. The Board of Trade carat was divisible into four "diamond grains", but measurements were typically made in multiples of carat. There were also two varieties of "refiners' carats" once used in the United Kingdom — the pound carat and the ounce carat. The pound troy was divisible into 24 "pound carats" of 240 grains troy each; the pound carat was divisible into four "pound grains" of 60 grains troy each; and the pound grain was divisible into four "pound
Carat (mass) suggested, and was finally proposed by the International Committee of Weights and Measures, and accepted at the fourth sexennial General Conference of the Metric Convention held in Paris in October 1907. It was soon made compulsory by law in France, but uptake of the new carat was slower in England, where its use was allowed by the Weights and Measures (Metric System) Act of 1897. In the United Kingdom the original Board of Trade carat was exactly grains (~3.170 grains = ~205 mg); in 1888, the Board of Trade carat was changed to exactly grains (~3.168 grains = ~205 mg).