Diamonds are sold by the carat (ct), not to be confused with karat (kt), which refers to gold purity. One carat weighs 2000 milligram. It should be noted that carat is a unit of weight, not of size. Many people would expect a 1 carat diamond to look the same size as a 1 carat ruby or a 1 carat emerald, for example. This is not the case, as ruby weighs more than diamond, while diamond weighs more than emerald.

Jewelers sometimes refer to the carat weight of diamonds in points, where one point = 1/100 of a carat. So a 25-point diamond is 0.25 carats in weight.

However, if properly cut, two round brilliant cut diamonds of the same carat weight will be the same diameter in size. Spread refers to the size the diamond appears to be, not how much it weights. If a diamond is cut shallow, it will appear larger (have a larger spread) than a diamond that is ideally cut.

The price per carat of diamonds is not proportionate - a 2 carat diamond doesn't cost twice as much as a 1 carat diamond, it will likely cost three times as much.