Diamonds are weighed in diamond carat weights. A carat weighs 200 milligrams. Diamond professionals also use the point system for smaller diamonds, with 100 points being equal to one carat.
A carat is usually abbreviated to ct.
Ct.tw. refers to total carat weight. It is the total weight of all the diamonds in a piece of jewelry. For example, in the case of diamond earrings, the total carat weight refers to the weight of the diamonds in both earrings together. If the total carat weight is 1 carat, each individual earring is set with a 1/2 carat diamond.
Two diamonds with exactly the same carat weight but with a different shape can have very different dimensions depending on their cut.
10.00 x 5.20 mm - One carat marquise diamond
6.30 x 6.30 mm - One carat round diamond
5.50 x 5.50 mm - One carat princess diamond
8.00 x 5.50 mm - One carat pear diamond