Diamond carat weight is the measurement of how much a diamond weighs. A metric "carat" is defined as 200 milligrams. This allows very precise measurements to the hundredth decimal place. The diamond price increases with carat weight, because larger diamonds are rarer and more desirable. But two diamonds of equal carat weight can have very different values and prices - depending on three other factors within the 4Cs: Clarity, Color, and Cut. It's important to remember that a diamond's value is determined using all of the 4Cs, not just carat weight.