Although commonly thought to refer to the actual size of a diamond, a carat is actually a standard unit of measure that defines the weight of a diamond. One carat is equivalent to 200 milligrams. Since it is a unit of measure and not size, two diamonds of the same carat weight may appear to be different sizes depending on how the diamond is cut. Some diamonds will have extra weight on the bottom part of the stone, and therefore appear smaller. A Hearts On Fire diamond is perfectly cut, and will appear larger than many diamonds of a heavier carat weight.
The metric carat, which equals 2 milligram, is the standard unit of weight for diamonds and most other gems. If other factors are equal, the more a diamond weighs, the more valuable it will be.
The weight of diamonds is measured in carats, a metric carat being a fifth of a gram.