Carat (mass)

An 'international carat' of 205 milligrams was proposed in 1871 by the Syndical Chamber of Jewellers, etc., in Paris, and accepted in 1877 by the Syndical Chamber of Diamond Merchants in Paris. A metric carat of 200 milligrams – exactly one-fifth of a gram – had often been suggested in various countries,[3] and was finally proposed by the International Committee of Weights and Measures, and unanimously accepted[3] 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.[13]

In the United Kingdom the original Board of Trade carat was exactly ​3 ¹⁶⁴⁷⁄₉₆₉₁ grains (~3.170 grains = ~205 mg);[lower-roman 2] in 1888, the Board of Trade carat was changed to exactly ​3 ¹⁷⁄₁₀₁ grains (~3.168 grains = ~205 mg).[lower-roman 3] 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,[lower-roman 4] but measurements were typically made in multiples of ​ ¹⁄₆₄ carat.

There were also two varieties of refiners' carats once used in the United Kingdom — the pound carat and the ounce carat.[lower-roman 5] 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 quarters of 15 grains troy each. Likewise, the ounce troy was divisible into 24 ounce carats of 20 grains troy each; the ounce carat was divisible into four ounce grains of 5 grains troy each; and the ounce grain was divisible into four ounce quarters of ​1 ¹⁄₄ grains troy each.[15]

The solidus was also a Roman weight unit. There is literary evidence that the weight of 72 coins of the type called solidus was exactly 1 Roman pound, and that the weight of 1 solidus was 24 siliquae. The weight of a Roman pound is generally believed to have been 327.45 g or possibly up to 5 g less. Therefore, the metric equivalent of 1 siliqua was approximately 189 mg. The Greeks had a similar unit of the same value.[16]

Gold fineness in carats comes from carats and grains of gold in a solidus of coin. The conversion rates 1 solidus = 24 carats, 1 carat = 4 grains still stand.[17] Woolhouse's Measures, Weights and Moneys of all Nations[18] gives gold fineness in carats of 4 grains, and silver in pounds[18] of 12 troy ounces of 20 pennyweight each.