Diamonds are sold by the carat (ct), which is a unit of weight, not size. One carat weighs .20 grams. Since larger diamonds are more rare than smaller diamonds, the price increases in larger proportions as the diamond gets larger.
The term “points” refers to a diamond’s carat weight. One point equals 1/100th of a carat. Therefore, a 1/2 carat diamond (.50) is referred to as 50 points.