The carat is a unit of weight used for the gems.
This small weight was based on the small seed from the Carob tree which was used as a measure of weight due to its uniform size. In 1907, the (metric) carat was defined as being equal to 200 milligrams (0.2g)
The carat also indicates the proportion of gold in an alloy. An alloy with 50% gold is called 12 carat ; pure gold is 24 carat. The caratage most used in France in high jewelry is 18 carat (or 750/000) that means 18 parts of pure gold and 6 parts of other metals.
