Diamonds are sold by carat, (ct.) not to be confused with karat, (kt.), which in the United States refers to gold quality. The carat is a unit of weight, not size. This is an important concept since the specific gravity varies among different minerals. For example, Emerald has a lower specific gravity than diamond. A one-carat emerald is 20% larger than a one-carat diamond. Ruby has a higher specific gravity than diamond, which makes a one-carat ruby smaller than a one-carat diamond.

Everyone first asks what size is the diamond? Most times color, cut, and clarity seem of secondary importance. It is very important to realize that small shifts in quality and size can make tremendous differences in value. As the size of the diamond increases, the price increases significantly. Diamonds weighing two-carats will sell for far more per carat than one-carat diamonds. As you will see in the chart below, the price of a diamond tends to increase exponentially as the size increases.