Caratage is the easiest of the 4 Cs to understand and measure, inasmuch as it refers to weight. Furthermore, because the density of diamonds is essentially constant, as the caratage of a diamond increases, so does its relative size. It is simple to obtain, because all that is required to weigh the diamond on a scale. Unlike the other three Cs – color, clarity and cut – it is not subject to human interpretation.

The term “carat” originates from the Greek word “karaton,” which was the carob seed that was used to balance scales in ancient times. A carat is equal to 200 milligrams.

It is important to distinguish between “carat,” which refers to the weight of a gemstone, and “karat,” which refers to a fineness of a precious metal.

Carats usually are expressed a full number or zero to two decimal points, such as 2.15 carats 0.60 carats. Hundredths of a carat are sometimes referred to as points, meaning that 100 points equals 1.00 carats.

Another term that is sometimes used is “grainer.” It dates back to the trade in the Far East, where diamonds were weighed against grains of rice, with 1.00-carat equaling four grains of rice.  Members of the trade will today refer to four grainers (1.00 carat), six grainers (1.50 carats) and eight grainers (2.00 carats). A one grainer could alternatively be referred to as 25 pointer or a 0.25-carat diamond.

The price of a diamond does not necessarily rise at rate that is consistent with the rise in number of carats. Larger diamonds are rarer, so they typically have a greater monetary value per carat. In other words, the value rises at a faster rate as it gets bigger in size. And there are other factors at play. For example, a 0.95-carat stone is 5 percent smaller than 1.00-carat stone, but it is more than 5 percent lower in value. Passing the 1.00-carat benchmark tends to boost a stone’s value, because of market demand. This does not exist between a 1.01-carat diamond and 1.02-carat diamonds, but it does between a 0.99-carat diamond and  a 1.00-carat diamonds.