Carat in mass

The carat (ct) is a unit of mass equal to 200 mg (0.00705 oz; 0.00643 ozt), which 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 100 points of 2 mg. Other subdivisions, and slightly different mass values, have been used in the past in different locations.

General information
Symbol ct
Conversions
1 ct in ... ... is equal to ...
milligrams 200
Conversions (imperial)
1 imp ct in ... ... is equal to ...
ounces 0.00705

In terms of diamonds, a paragon is a flawless stone of at least 100 carats (20 g). The ANSI X.12 EDI standard abbreviation for the carat is CD.

Etymology
First attested in English in the mid-15th century, the word carat comes from Italian carato, which comes from Arabic (qīrāṭ; قيراط), in turn borrowed from Greek kerátion κεράτιον 'carob seed', a diminutive of keras 'horn'. It was a unit of weight, equal to 1/1728 (1/123) of a pound (see Mina (unit)).

Definitions

Gemological carat (mass)
Unit of mass used primarily for weighing gems and pearls, and abbreviated ct. In this sense, a carat historically represented one hundred and fortieth (1⁄140) of an ounce (205 milligrams). Since 1907, with the introduction of the decimal metric system, the " Decimal Carat " has been used, which represents a mass of 200 milligrams (0.2 grams), therefore 5 carats are equivalent to 1 gram; 10 carats, to 2 grams; 100 carats, to 20 grams.

Goldsmith's carat (purity)
Designates the purity of the gold used in jewelry. In this sense, a karat (abbreviated K or kt) represents one twenty-fourth (1⁄24) part of the total mass of the alloy that makes up the metal (approximately 4.167%). For example, if a jewel made of 18 karat gold, its alloy is made of 18⁄24 (or what is the same 3⁄4) parts of gold and has a purity of 75%, while a 24 karat piece is made of 24⁄24 parts of gold and is therefore pure gold.

History
Originally, the carat was the weight of a dried seed of the carob tree (Ceratonia siliqua). This was supposedly due to their uniform size and uniform weight of 0.2 g on average. In the Middle Ages, one carat was equivalent to the weight of three grains of barley or four grains of wheat.

Carob seeds have been used throughout history to measure jewelry, because it was believed that there was little variance in their mass distribution. However, this was a factual inaccuracy, as their mass varies about as much as seeds of other species.

In the past, each country had its own carat. It was often used for weighing gold. Beginning in the 1570s, it was used to measure weights of diamonds. Until the beginning of the 20th century, the carat was defined differently from country to country. To create uniformity and obtain a "smooth" value in grams, the metric carat (Carat métrique) was defined with its current value in 1907.

Standardization
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 is exactly one-fifth of a gram and had often been suggested in various countries, and was finally proposed by the International Committee of Weights and Measures, and unanimously 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.

Historical definitions
Carat before 1907

Location	mg
Cyprus	187
Brazil	192.2
Egypt	195
Ambonia	197
Florence	197.2
International carat Batavia, Borneo, Leipzig	205
South Africa (1)	205.304
London-New York (1)	205.303
Spain	205.393
London-New York (2)	205.409
Berlin	205.44
Paris, East India	205.5
South Africa (2)	205.649
Amsterdam	205.7
Lisbon	205.75
Frankfurt (on Main)	205.77
Vienna	206.13
Venice	207
Madras	207.353
Bucharest	215
Livorno	215.99

UK Board of Trade
In the United Kingdom the original Board of Trade carat was exactly 3+1647⁄9691 grains (~3.170 grains = ~205 mg); in 1888, the Board of Trade carat was changed to exactly 3+17⁄101 grains (~3.168 grains = ~205 mg). Despite it 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.

Refiners' carats
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.

Greco-Roman
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. The conversion rates 1 solidus = 24 carats, 1 carat = 4 grains still stand. Woolhouse's Measures, Weights and Moneys of All Nations gives gold fineness in carats of 4 grains, and silver in troy pounds of 12 troy ounces of 20 pennyweight each.

Average size-mass equivalence
A one-carat brilliant-cut diamond has a diameter of approximately 6.5 mm. However, the size varies depending on the type of gem, each having its own density.

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