Dealers can estimate correct weights of given stones by eye alone
It is remarkable, however, how many dealers in gem stones can estimate almost correct weights of given stones by eye alone. This applies particularly to brilliants. Daily practice has given them this skill in judgment, the accuracy of which is only exceeded by direct weighing. This skill is of great assistance when one is called upon to value a piece of jewelry which contains a number of various size stones.
Many of the old large diamonds have been known by their weight alone over a great many years, although a universally recognized unit was not always in use. We find various terms describing weights of gem stones in old works, and this has sometimes resulted in confusion. Perhaps the first unit of weight was the small, hard, dried seed of the locust tree, which was found throughout the East and in the Mediterranean countries. The size of this seed was fairly constant, and it was therefore something which could be used as a unit of comparison. In western Europe, the barley seed was used, and a Greek word for the shape of the pods is the derivation of the word carat.