A carat, as a unit of weight, originally comes from the Arabic qirat which was roughly the weight of 4 grains and was traditionally used to weigh small amounts.

Dictionary Definition

A unit of weight in gemstones, 200 milligrams (about 3 grains of troy or avoirdupois weight).
Abbreviation: c., ct.

Originally, carats weights were only used to measure diamonds but carats have now become the accepted unit of measurement for gemstones as well.

Gemstones Weight

The weight of a gemstone does not necessarily correspond to its size. The reason for this is that difference gemstones have different densities. A one carat round cut ruby will not have exactly the same measurements as a one carat round cut sapphire or emerald because of their different relative densities.

Carat Total Weight

Total carat weight (cttw) is the weight of all the gemstones in an item of jewelry added together.

Definition courtesy of Random House Unabridged Dictionary, Random House, Inc. 2006.