CARAT WEIGHT

Diamonds are sold by the carat (shown as ct.), which is actually a unit of weight, though most think of a carat in terms of size. The word "carat" comes from the "carob" seed, the original unit of measure for diamond traders. Today, a carat is equal to exactly 0.2 grams (about the weight of a paper clip). One carat weighs 1/5 of a gram and is divided into 100 points, so a diamond weighing 1.10 ct. is referred to as "one carat and ten points."
For example

Carat	Points
3/4 carat	75 pointes
1/3 carat	30 pointes
1/5 carat	20 pointes

It is important to note that diamonds of the same weight don't necessarily have the same size appearance. Those cut too shallow or deep may look small for their weight, or suffer in brilliance.

Usually, as the carat size of a diamond increases, the diamond's price increases at an increasing rate - the larger the diamond, the more increasingly rare it is. When diamonds are mined, large gems are discovered much less frequently than small ones, which make large diamonds much more valuable.