In ancient times gem traders would use a method of weight based on the carob seed. This seed from the locust tree is very consistent in its weight and made an idea basis of measurement. A one carat diamond used to equal one carob seed but today this translates to 200 milligrams or 1/5 of a gram. Diamond weights today are expressed in "points". A one carat diamond equals 100 points. A fifty point diamond equals half a carat, and a stone that weights 150 points is a carat and a half.

Most people believe that the bigger the diamond the better. However, as explained in the section on cut a stone that appears larger may not be heavier. Therefore, you should be careful in selecting diamonds solely based on the weight of the stone.

A diamonds value will grow dramatically if it crosses the threshold from say 0.99 to 1.00 carats. Though only one point separates the two diamonds the one carat stone will be significantly more money. It doesn't take many people long to figure out that they might be able to save money by buying under these margins.