Carat (unit)

The carat (ct) 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.

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.

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 though it was not likely used to measure gold in classical times.

It is commonly believed that carob seeds were used because their mass does not vary much. But in fact, they vary about as much as seeds of other species.

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.

In the United Kingdom the original Board of Trade carat was exactly  3 ¹⁶⁴⁷⁄₉₆₉₁ grains (~3.170 gr); in 1888, the Board of Trade carat was changed to exactly  3 ¹⁷⁄₁₀₁ grains (~3.168 gr). Despite its being a non-metric unit, a number of metric countries have used this unit for its limited range of application.

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