The weight of the diamond is measured in carat weight, measured at the GIA (Geemological Institute of America ™) Laboratory, usiong a highly precide electronic scale and then rounded to the nearest hundreth of a cart. One carat is equal to 1/5th of a gram. Carat weights are also sometimes quoted in "points" (hundredths of a carat). A "50 point" diamond, for example, weighs ½ of a carat. Carat-weight is the easiest of the characteristics of diamond to determine. Two diamonds of equal weight can, however, have very unequal value. That inequality depends on their cut, color and clarity. Fine quality can be found in diamonds of all shapes and sizes, but since larger diamonds are found in nature much less frequently, they are generally more valuable than smaller stones. A two-carat diamond of a given quality is therefore always more expensive than two one-carat diamonds of the same quality.

To get a sense for the relative size of round diamonds, look at the chart at the right. (Remember, this chart is not to scale – the size will depend on the resolution of your monitor!).