The carat is a unit of weight for gemstones, evidently this includes diamonds. One carat has been standardized as the equivalent of exactly 0.2 grams. As the carat weight increases, generally the value of the diamond will increase exponentially due to its rarity. The word ‘carat’ supposedly has been derived from the Ancient Greek word for the seed of the carob tree ‘keration’. Because the weight of these seeds is fairly constant, these have been used in earlier times as weights for precision measurements (of diamonds for example).