Derived from the word carob, a carat is a unit of weight used to measure diamonds, standardized at 200 milligrams. Years ago, a 1-carat diamond was equal to the weight of one bean from the carob tree. You might also see diamond weight expressed in points, with one carat equal to 100 points (a 75-point diamond weighs 3/4 carat, and so on).

Larger, weightier diamonds are considered rare and more valuable. However, a stone's value is not determined by carat weight alone — the three other C's must also be considered. For instance, a heavily included 2-CT diamond may not be worth as much as an internally flawless but smaller stone.