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,[1][2] and soon afterwards in many countries around the world.

[4] 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',[5][6][7] a diminutive of keras 'horn'.

[6][9][5] Carob seeds have been used throughout history to measure jewelry, because it was believed that there was little variance in their mass distribution.

A metric carat of 200 milligrams is exactly one-fifth of a gram and had often been suggested in various countries,[2] and was finally proposed by the International Committee of Weights and Measures, and unanimously accepted[2] 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.