Diamond weight is measured in Carats, a small unit of measurement equal to 200 milligrams. Each carat is divided into 100 points. Therefore, a half-carat stone may be referred to as a "50-pointer" or "50-points." Carat weight is the easiest of the 4 C's for gemologists to determine because of the use of sophisticated measuring equipment.

Two diamonds of equal carat weight might vary greatly in value depending upon their cut, color and clarity. This is important because when mounted, one diamond may appear larger than the other, although they actually weigh the same. Because large diamonds are rare, they generally have a greater value per carat. For example, the price of a two-carat stone will be several times higher than four 50-pointers of equal quality.

Diamonds are sold by carat weight. In the diamond trade, price is given in price per carat and not in total dollars. For example a 1.02ct diamond that costs $2,000 per carat would cost 1.02 X $2,000 (= $2,040).

The proper dimension of a diamond for its weight is an extremely crucial element in its value and desirability.  For example, a 1.00 carat round diamond should measure approximately 6.5mm in diameter, a 1.50ct should measure around 7.5mm, a 2.00ct should measure around 8.1-8.2mm, and so on.  If you see a diamond listed on line in the 1.00ct size which only measures 6.2-6.3mm (or smaller), you should not buy it!  That diamond doesn't look any larger than a well cut 0.80-0.90ct diamond.  This also applies to 1.50ct diamonds which measure 7.2 or 7.3mm.  These diamonds may have a good grade on paper, even a terrific cut grade and a "great price" but it is not a diamond I would ever recommend to my private customer.   (See The A. Fishman & Son Cut Diamond)  The same factor applies to fancy shaped diamonds with the additional requirement that the diamond's shape has to be right, not just its dimensions.