Carat is the weight of the diamond measured in carats.

One carat is divided into 100 "points," so that a diamond of 75 points weighs 0.75 carats.

One carat (ct.) equals 1/5th of a gram.

Diamonds are measured by carat weight, and not size. Two diamonds of the same weight can have different sizes. A shallow (or flat) cut diamond will have a larger diameter, but shallow depth and look larger. Similar a deep cut (or pointed) diamond will have a smaller diameter and deep depth thus appearing smaller.

So cut is important. The diamond should be cut to proper proportions, so the light reflection is maximized.

A diamond of 1/2 a carat has a approximate diameter of 5.2mm , one of 1 carat 6.5mm and one of 2 carats 8.2mm. Although the weight of a 0.50 carat is half that of a carat diamond the size difference is only 20%, this is because most of the weight of a diamond is at the bottom part of the stone. The larger diamonds are much rarer and their value increases with size.