The "C" called carat refers to
the weight of a diamond. It is derived from "carob" seeds which were originally used to weigh diamonds because of their precise weight of 0.2gr which is the equivalent weight of a 1 carat diamond.

Diamonds range in size from any fraction of a carat to several carats in size. The carat scale is based on a metric system of 100 points = 1 carat. In other words fractions of 1 carat are reflected as diamonds weighing a fraction of 1. For example a diamond weighing 50 points is measured as 0.50ct.

Given the rarity of larger stones, a diamond's cost increases considerably as the size goes up.

The world's largest cut diamond is an unnamed Fancy Black, containing small red diamond crystals. It weighs 555.55 carats and was polished into 55 facets over several years and completed in June 2004.