Carat. Goldsmiths and assayers divide the troy pound, ounce, or any other weight into 24 parts, and call each a carat, as a means of stating the proportion of pure gold contained in any alloy of gold with other metals. Thus, the gold of our coinage and of wedding-rings, which contains of pure gold, is called '22 carats fine,' or 22-carat gold. The lower standard, used for watch-cases, &c., which contains of pure gold, is called 18-carat, and so on. The carat used in this sense has therefore no absolute weight; it merely denotes a ratio. This, however, is not the case with the jewelry carat used for weighing diamonds and other precious stones, pearls, &c., which has a fixed weight, equal to troy grains, and is divided into quarters, or 'carat grains,' eighths, sixteenths, thirty-seconds, and sixty-fourths. These carat grains are thus less than troy grains, and therefore the jeweller has to keep a separate set of diamond weights. Even the carat with fixed weight varies in various countries—from about 216 milligrams in Leghorn to 195 in Florence. In England it is 205.4090 milligrams; in France, 205.5000.
The name seems to have come through the Arabic qirrat from the Greek keration, the fruit of the Carob (q.v.) or locust-tree (Ceratonia siliqua), used also as a weight. The origin of the word has also been sought in Kuara, the native name of the Erythrina Abyssinica, or coral-tree, the seeds of which, it is said, were very equal in size, and were used for weighing gold and precious stones.