Diamonds are sold by the carat (shown as ct.) which is a unit of weight and does not directly relate to height or size. The word carat comes from the carob seed, the original unit of measure for diamond traders. One carat is equal to exactly 0.2 grams.
Carat weight is unrelated to the similar sounding karat, which refers to gold’s purity. One Carat is comprised of 100 points and sometimes weights can be quoted as points for example 75 points is the same as ¾ Carats.
Two diamonds of equal carat weight can have very different costs as a result of other factors (cut, colour and clarity). As the carat size of a diamond increases, the diamonds price increases at an increasing rate. Why? Because the larger the diamond, the more increasingly rare it is. Less than one in one million rough stones mined are large enough to produce a one carat polished diamond. As carat weight increases, typically you will pay more not only in total but also on a price per carat basis.