The size of a diamond is defined by its weight, which is expressed in carats.  A carat is further subdivided in 100 points (0.01 carat = l point). One carat is equal to 0.20 grams. The value per carat increases with carat size, because larger rough diamonds occur less frequently.  In other words, 2 half-carat diamonds taken together will not cost as much as 1 one-carat diamond, as the one-carat stone is rarer.