Which unit is used for measuring gemstones and pearls?

The carat (ct) is a unit of mass equal to 200 mg (0.00705 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.

First attested in English in the mid-15th century, the word "carat" comes from Italian "carato", which comes from Arabic "qīrāṭ", in turn borrowed from Greek "kerátion" "carob seed".

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.