A diamond’s weight, and therefore its size, is expressed in carats.
In ancient times merchants used the carob seed as a base unit of weight when assessing diamonds. This unit of measurement has become what is commonly known as a carat, and has a weight of 200 milligrams.
For diamonds under one carat, each carat is divided into 100 points, like cents in a dollar, 0.5 carat is 50 points (The scale shown here is indicative only.) The larger the diamond the rarer it is found in nature, and usually the more expensive it is to purchase. But that is not always the case and size is only one of the considerations of value.