What is the Carat ?

Source: en.wikipedia.org

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