Carat is a measurement of weight, with one diamond carat equal to 200 milligrams. The term is derived from the word ‘carob’ because in the ancient world, carob seeds were used as a reference for diamond weight. Precise measurements are taken to the hundredth decimal place to ensure complete accuracy. As large diamonds are more rare and desirable, they are more valuable, but two diamonds of equal carat weight may have different values based on the other factors.