Carat is often confused with size even though it is actually a measure of weight. The cut of a diamond can make it appear much larger or smaller than its actual weight.
High carat diamonds often appear to be brighter or more brilliant than their lower carat siblings due to the prism effect of light traveling over larger distances.
One carat (not to be confused with Karat—the measure of purity of gold) is the equivalent of 200 milligrams. One carat can also be divided into 100 "points." A .75 carat diamond is the same as a 75-points or a 3/4 carat diamond.
The term carat is a derivative of the word carob. Carob seeds, which are surprisingly uniform in weight, were used as a reference for diamond weight in ancient civilizations. One carob seed equaled one carat.
It is often assumed that a 1-carat diamond costs exactly twice the price of a 1/2-carat diamond. This is not the case. Since larger diamonds are found less frequently in nature, a 1-carat diamond will cost much more than twice as much as a 1/2-carat diamond, assuming color, clarity and cut remain constant.
The Hancock Diamond, weighing less than one carat, famously sold for $880,000 at Southeby's in 1987, due to its very rare red color.
