The carat is a unit of weight, not size. One carat (1 ct) weighs 1/5 of a gram and is divided into 100 points.. Carat weight is the most obvious factor in determining the value of a diamond, but it is important to bear in mind that two stones of equal size can have very different values depending on their quality. It is also important to bear in mind that carat weight does not affect the value in diamonds proportionally, and the larger the stone, the more disproportionate the increase in cost per carat.