Diamond Size and Diamond Carat Weight
The size of a diamond is proportional to its carat weight. When rough diamonds are cut and polished into finished diamonds, as much as 2/3 of the total carat weight may be lost. Since larger rough gems of high quality are found less frequently than smaller rough gems of high quality, a single two carat diamond will be more expensive than two one –carat diamonds of the same quality.
In the United States, the majority of diamonds used in jewelry and sold as loose diamonds are one carat or less in weight. The average engagement ring diamond sold in the U.S. is less than 1/2 carat in weight.
A diamond will increase in weight much faster than it increases in actual "face-up" diameter. For example, while an ideal cut one–carat diamond measures approximately 6.5mm in width, a diamond of twice its weight measures only 8.2mm wide—less than a 30% increase.