Of the 5 C's, carat weight is the simplest factor used to determine the value of a diamond. As with all precious gems, the weight of a diamond is expressed in carats. One carat equals one-fifth of a gram or 1/142 of an ounce. One carat is divided into 100 points, so a 3/4 carat diamond weighs 75 points or .75 carats.

As diamonds increase in weight, they become much  rarer. Their price per carat usually increases geometrically, not arithmetically. Thus, for example, a two-carat diamond  will usually be far more than double the price of a one-carat diamond of the same quality.

Diamonds are accurately weighed on a calibrated scale when they are loose, or free from any mounting.