BijanDiamonds.com Dictionary
Carat:
Carat (ct.) is the standard unit of weight used for gemstones. 1.00 carat is equal to 0.20 grams or 200 milligrams.

Diamonds are weighed to a thousandth (0.001) of a carat and then rounded to the nearest hundredth, which is called point. Over a carat, diamond weights are usually expressed in carats and decimals. A 1.05 ct. stone, for example, would be described as "one point oh five carats" or "one oh five." A diamond that weighs 0.73 ct. is said to weigh "seventy-three points," or an "seventy-three pointer."