The diamond is sold by its weight, estimated in carats,—a carat being equal to three and one fourth grains Troy, and subdivided into half, quarter, eighth, and so on. It is difficult to say what a rough diamond is worth, since a great many reasons may occur to cause its fluctuation in value. The ordinary estimate for a cut diamond is sixty dollars a carat, that is to say, when the stone weighs a single carat. The price of the diamond exceeding a carat is not in proportion to its weight, but by the square of the weight, that is to say, to the weight multiplied by itself. Thus, if a diamond weighing one carat is worth sixty dollars, one which weighs two carats is worth 2 × 2, and then multiplied by sixty, or two hundred and forty dollars. A stone of three carats is worth 3 × 3, multiplied by sixty, or five hundred and forty dollars. The value, therefore, of a polished diamond is found by multiplying the square of the weight by the price of a stone of one carat. This is the rule generally given for the pricing of diamonds; but the value of each stone varies more or less according to its character, so that one stone weighing fifteen carats might be worth three or four thousand dollars more than another stone of the same weight. The best rule, probably, for obtaining the price of diamonds, is to ask a man who has them for sale. Diamonds are sold very much like any other commodity, that is, for the highest price the purchaser is willing to pay. Diamonds, especially those of a large size, require, it is needless to say, great care in keeping, to save them from being stolen. The crown jewels of England are kept in the Tower of London in an iron cage surrounded with glass. Some of them are of great antiquity. The crown jewels include not only diamonds, but some valuable rubies, sapphires, emeralds, and pearls. The crown contains a heart-shaped ruby, which is said to have been given to Edward the Black Prince by Don Pedro, King of Castile, after the battle of Najera, A. D. 1367. [Sidenote: FAMOUS STONES.] It was afterwards worn in the helmet of Henry V., at the battle of Agincourt, in the year 1415. The crown jewels of France disappeared in 1792, during the troubles of the first republic, though they were kept under seal, and in the royal treasury. Some of them were afterwards found buried in an obscure place, which was named in an anonymous letter sent to the prime minister. The famous Regent diamond was in this casket.