MALACHITE. Malachite is found in many copper mines, but principally in those of the Ural Mountains. AZURITE. Azurite is found in the Arizona mines and in Chessy, in France (hence the name chessylite, sometimes used instead of azurite). * * * * * REFERENCES. Students who wish to get a fuller account of the occurrence of precious stones should run through G. F. Herbert-Smith's _Gem-Stones_ under the different varieties. This work is the most recent authentic work of a strictly scientific character. Dr. George F. Kunz's _Gems and Precious Stones of North America_ gives a detailed account of all the finds in North America up to the time of publication. Many of these are of course of little commercial importance. The _Mineral Resources of the United States_ contains annually a long account of the occurrences of gem materials in this country. A separate pamphlet containing only the gem portion can be had gratis from the office of the United States Geological Survey, Washington, D. C. LESSON XXII HOW ROUGH PRECIOUS STONES ARE CUT ROUGH PRECIOUS STONES. John Ruskin, who had the means to acquire some very fine natural specimens of gem material was of the opinion that man ought not to tamper with the wonderful crystals of nature, but that rather they should be admired in the rough. While one can understand Ruskin's viewpoint, nevertheless the art of man can make use of the optical properties of transparent minerals, properties no less wonderful than those exhibited in crystallization, and indeed intimately associated with the latter, and, by shaping the rough material in accordance with these optical properties, greatly enhance the beauty of the gem. No material illustrates the wonderful improvement that may be brought about by cutting and polishing better than diamond. In the rough the diamond is less attractive in appearance than rock crystal. G. F. Herbert-Smith likens its appearance to that of soda crystals. Another author likens it to gum arabic. The surface of the rough diamond is usually ridged by the overlapping of minute layers or strata of the material so that one cannot look into the clear interior any more than one can look into a bank, through the prism-glass windows that are so much used to diffuse the light that enters by means of them. Being thus of a rough exterior the uncut diamond shows none of the snap and fire which are developed by proper cutting. As the diamond perhaps shows more improvement on being cut than any other stone, and as the art of cutting the diamond is distinct from that of cutting other precious stones, both in the method of cutting and in the fact that the workers who cut diamonds cut no other precious stones, it will be well to consider diamond cutting separately. Before discussing the methods by which the shaping and polishing are accomplished let us consider briefly the object that is in view in thus altering the shape and smoothing the surface of the rough material.