As with all precious stones, the weight of a diamond is expressed in carats. Size alone does not result in beauty or value, but it is an important factor to be considered when purchasing a diamond.
The math of carats
Carat is a unit of weight measurement equal to one fifth of a gram (1 carat = 0.2 grams). The carat measurement is divided into 100 parts called "points" (1 carat = 100 points). Therefore, a 25-point diamond is referred to as a "quarter carat" (0.25 carat) and a 50-point diamond is referred to as a "half carat" (0.50 carat).
Relationship of carat and cost
Diamonds, when mined, are most often found in small pebble form. Larger diamonds are more rare and therefore result in a greater value for their size. The cost of diamonds increases exponentially as the size increases. In other words, the cost of a two-carat diamond will be much greater than the total cost of two, one-carat diamonds of the same quality.
Relationship of carat and cut
It should be remembered that carat is a unit of weight measurement and not a measurement of a stone's diameter. For example, it is possible to have two diamonds each with a round shape weighing 1.00 carat, but each having different diameters and the appearance of being different in size. This is because a diamond is three-dimensional and the carat weight can be found not only in the diameter of the stone, but also in its depth. A one-carat diamond, if cut with a shallow depth, may have a diameter of over 7.0 millimeters, or if cut with a heavy depth may have a diameter of under 6.0 millimeters. Although the novice diamond buyer may assume that the diamond with the greater diameter is to be preferred, this is not the case because either extreme of a deep cut or a shallow cut will result in decreased brilliance (please see "cut" for additional information on how the proportions of the diamond will affect its beauty).