15. The precious stones generally have likewise been imitated with great success. The basis of the different compositions is a _paste_ made of the finest flint glass, the materials of which have been selected and combined with great care. The desired color is produced with metallic oxydes. A great number of complex receipts are in use among manufacturers of these articles. 16. _The Lapidary._--The precious stones and imitations of them in glass are brought to the desired form by the lapidary. The instrument with which this artist chiefly operates is a wheel which is made to revolve horizontally before him. It is put in motion by means of an endless rope extending from another wheel, which is moved with the left hand of the operator, while, with his right, he holds, in a proper position, the substance to be reduced. 17. The precious stones, being of small size, cannot be held with steadiness on the wheel with the hand, nor with any holding instrument; they are, therefore, first fastened, by means of sealing-wax, to the end of small sticks. By this simple means, and a small upright post, against which the hand or the other end of the stick is rested, the workman can hold a stone in any position he may desire. 18. The lapidary's wheel is made of different kinds of metals. The diamond is cut on a wheel of soft steel, by the aid of its own dust mixed with olive oil. The Oriental ruby, sapphire, and topaz, are cut on a copper wheel in the same manner, and polished with tripoli and water. Stones of a less degree of hardness are cut and polished on a leaden or tin wheel with emery and rotten stone. 19. The ancients were not acquainted with any method of cutting the diamond, although they applied its powder to polishing, cutting, and engraving other stones. Gems of this kind, either rough, or polished by nature, were set as ornaments, and were valued according to the beauty and perfection of their crystallization and transparency. The value of any precious stone, or jewel, depends much upon the skill of the lapidary. 20. _The Pearl-fisherman._--Pearls are obtained from a testaceous fish of the oyster kind, found in the waters of the East and West Indies, as well as in other seas of different latitudes. These oysters grow in some parts of the globe, in clusters, on rocks in the depths of the sea. Such places are called _pearl-banks_, of which the most celebrated are near the islands of Ceylon and Japan, and in the Persian Gulf. The finest and most costly pearls are the Oriental. 21. Pearls are considered by some to be morbid concretions, or calculi, produced by the endeavor of the animal to fill up the holes which may have been made from without by small worms. Others suppose them to be mere concretions of the animal juice about some extraneous matter which may have been intruded by some means into the shell.