Gemstone Size and Weight
A carat, as a unit of weight, originally comes from the Arabic qirat
was roughly the weight of 4 grains and was traditionally used to weigh small amounts.
A unit of weight in gemstones, 200 milligrams (about 3 grains of troy or avoirdupois
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.
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
Definition courtesy of Random House Unabridged Dictionary,
© Random House, Inc. 2006.