Carat is the unit of weight used to measure diamonds and precious gemstones. One carat is equal to 200 milligrams, or 0.2 grams.
The term carat originated from the Greek word "keraton",  or the "fruit of the carob", which refers to the seeds borne by the carob tree that grew throughout the Mediterranean region. Carob seeds were thought to be uniform in size and weight, and consistently weighed about 200 milligrams. The modern metric carat was adopted in 1913 in the United States, and this rapidly spread throughout the rest of the world as the standard unit of measurement for diamonds, gemstones and natural pearls.
Carat weight can be expressed with several different terms or abbreviations. 'Cttw',' ct.tw', 'carat total weight' and 'total carat weight' all refer to the total carat weight of all the diamonds in a ring combined. 'Ct.', 'cts' or 'carats' refers to the carat weight of a particular diamond.
See Also: Karat, Diamond, Gold