The weight of diamonds is measured in carats. A carat is one fifth of a gram.
Since larger stones are harder to find than smaller stones, the bigger diamonds sell for more money per carat.
One carat diamonds will sell for more on a per carat basis than one-half carat stones, with all other elements being equal.
According to Federal Trade Commission rules, it’s an unfair trade practice to misrepresent the weight of a diamond, and any weight quoted to you has to be correct within .005 carats.