A carat is a unit of weight used to measure the size of a gemstone such as a diamond. A karat is a measurement indicating the proportion of gold in an alloy out of 24 parts, so 18K gold is 18/24 parts gold. However, the use of carat (in place of karat) to indicate the fineness of gold is also considered acceptable.

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.