CARAT

The carat is a unit of weight for precious stones: one carat equals 0.2 grams (it derives from the carob seed, once used as a weight standard). The carat measures mass, not size, so stones of equal weight can look different depending on cut and density. Carat is one of the well-known 'four Cs' of diamond grading (carat, cut, colour, clarity). It is important not to confuse the carat of a stone's weight with the karat of gold purity (there a karat is the fineness of the alloy). As weight increases, a stone's price rises not linearly but in jumps, because large clean stones are far rarer. Carat is a key indicator of a gem's value.