The carat is a unit of mass equal to 200 mg (0.2 g; 0.007055 oz) and is used for measuring gemstones and pearls. The current definition, sometimes known as the metric carat, was adopted in 1907 at the Fourth General Conference on Weights and Measures, and soon afterwards in many countries around the world. The carat is divisible into one hundred points of two milligrams each. Other subdivisions, and slightly different mass values, have been used in the past in different locations.
First attested in English in the mid-15th century, the word carat came from Middle French carat, in turn from Italian carato, which came from Arabic qīrāṭ (قيراط), which came from Greek kerátion (κεράτιον) meaning carob seed (literally "small horn") (diminutive of - keras, "horn") and was a unit of weight though it was not likely used to measure gold in classical times. The Latin word for carat is siliqua. This common belief that carat derives from carob seeds stems from the assumption that the seeds had unusually low variability in mass.
However, one group of researchers have found that carob seeds in fact have typical variability compared to the seeds of other species.
This was not the only reason. It is said that, in order to keep regional buyers and sellers of gold honest, potential customers could retrieve their own carob seeds on their way to the market, to check the tolerances of the seeds used by the merchant. If this precaution was not taken, the potential customers would be at the mercy of "2 sets of carob seeds". One set of "heavier" carob seeds would be used when buying from a customer (making the seller's gold appear to be less). Another, lighter set of carob seeds would be used when the merchant wanted to sell to a customer.
In the past, each country had its own carat. It was often used for weighing gold. Starting in the 1570s, it was used to measure weights of diamonds.
Historical definitions in the United Kingdom
Board of Trade carats
In the United Kingdom the original Board of Trade carat was exactly 3 1647⁄9691 (≈ 3.170) grains; in 1888, the Board of Trade carat was changed to exactly 3 17⁄101 (≈ 3.168) grains. 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 1⁄64 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 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.
The carat of the Romans and Greeks
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.
Gold fineness in carats comes from carats and grains of gold in a solidus of coin. One solidus = 24 carats, 1 carat = 4 grains, is preserved right up to this day. Woolhouse's Measures, Weights and Moneys of all Nations gives gold fineness in carats of 4 grains, and silver in (pound) of 12 ounces each 20 dwt.