What is a "carat" in gemmology ?

Carat : definition

The carat (noted ct) is the unit of mass and measurement used in jewelry for gemstones, it is directly related to the mass in grams since one carat is equivalent to 0.2 g and therefore 5 carats are worth 1 gram.

The carat also refers to the proportion of gold in an alloy : a 50% alloy of 12 carat grade gold, an alloy of 75% 18 carat grade gold and 24 carat pure gold.