Diamonds like most gems are weighed in metric carats.

Carat is a measure of weight though it is often confused with size. One carat is equal to 200 milligrams. One carat can also be divided into 100 points. A 0.75 carat diamond is the same as a 75-point or 3/4 carat diamond. Some people wrongly assume that the term point refers to the number of facets on the diamond.

People often think that a large diamond, 1.00 carat for example, would be twice the value of a 0.50 carat diamond. In fact, since larger diamonds are much more rare, a 1.00 carat diamond may cost more than twice the value of a 0.50 carat diamond, assuming the cut, clarity and colour are identical.

Because of the rarity and value of diamonds, the diamond industry weighs diamonds to a thousandth of a carat and rounds them to the nearest hundredth or point.

To appreciate how precise the measure is, we need to consider that
one-hundredth of a carat, or one point, is less than one fourteen-thousandth of an ounce.