Although it is extremely important to consider clarity, colour and cut when purchasing a diamond, carat weight is the "C" most likely to determine a diamond's value. Fortunately, it is the easiest characteristic to measure.
Most people think of a carat in terms of the size of the diamond, but it is actually based on weight. The weight of a diamond is traditionally measured in terms of carat, a unit of weight with 1 carat equaling .20 grams. Each carat is divided into 100 points, so a half carat is 50 points and is expressed as .50ct
Two diamonds of equal carat weight will have significantly different values because of the other factors: cut, colour and clarity. As a diamond increases in size and carat weight, the price will rise at an increasing rate because larger diamonds are more rare and therefore more valuable.