Carat
A diamonds size is measured in carat weight, and each carat is equal to 100 points.  A .75 carat diamond is the same as 75 points or a 3/4 carat diamond.  Since larger diamonds are rarer than smaller diamonds, the value rises exponentially with carat weight.

Carat is often confused with size even though it is a measure of weight.  The cut of a diamond can make it appear larger or smaller than its actual weight.

Diamond Relative Carat / Size Comparison