The weight of a diamonds is generally given in carats. The term carat originated in ancient times when gemstones were weighted against the carob bean. Each bean weighed about one carat. In 1913, carat weight was standardized internationally and adapted to the metric system. One carat equals 0.2 grams (200 milligrams) - a little more than 0.007 ounce. In other words, it takes 142 carats to equal 1 ounce.
The weight of small diamonds is frequently expressed in points, with one point equaling 0.01 carats. For example, five points is a short way of saying 5/100 of a carat.
Larger diamonds are much more rare than smaller ones. In fact, hundreds of tons of rock and ore must be processed to uncover a single one-carat gem quality diamond.
Since a carat is a unit of measure and not size, two diamonds of the same carat weight may appear to be different sizes depending on how the diamond is cut. Some diamonds will have extra weight on the bottom part (or pavilion) of the stone, and therefore appear smaller.