Size is the most obvious factor in determining the value of a diamond. Larger diamonds are much rarer than smaller diamonds. Prices rise exponentially with the carat weight. Diamonds have "special points:" .25ct, .50ct, .75ct, 1.00ct and so on. Every time a diamond hits a special point the price goes up.
With these special points, take into consideration that a .95ct diamond should be 6.4mm where as a 1.00ct should be 6.5mm. Can you tell the difference? Visually, the difference is minor; but in terms of cost, the difference can be dramatic.
As with all precious stones, the weight, and therefore the size of a diamond, is expressed in carats. One carat is divided into 100 points so that a diamond of 150 points weighs 1.5 carats.
Diamonds of equal size can have very unequal values. Depending on how the diamond is cut, a 1.10ct diamond could look like a .80ct. Carat size usually ranges on an average millimeter size per weight. Be sure to know the average of your carat size before you make your purchase. A 1.00ct diamond has an average millimeter size of 6.5.