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. Carat weight is unrelated to the similar sounding karat, which refers to gold's purity.
The weight of a diamond is traditionally measured in terms of carat, a unit of weight with 1 carat equalling .20 grams. Each carat is divided into 100 points, allowing for a very precise measurement of the diamond. This allows very precise measurements to the hundredth decimal place.
For instance a diamond that weighs .50 carat may be referred to as a “half-carat” or a “fifty-pointer”.
Two diamonds of equal carat weight will have significantly different costs because of the other factors: cut, color and clarity. As a diamond increases in size and carat weight, the price will increase at an increasing rate because larger diamonds are more rare and thus more valuable.