MINERALS. 148 Nickel is a metal which does not readily tarnish. It is extensively employed for electro-plating iron and other readily-oxidizable metals. number of valuable alloys, one of which is Ger- man silver. Nickel is employed to some extent in coinage. Antimony is a metal which when alloyed with lead is extensively employed as type- metal. .This alloy possesses the property of tak- ing sharp casts, thus permitting the type to be readily made. Valuable deposits of its ore occur in the United States and in Europe. Aluminium is a remarkably light metal, ob- tained from clay. It is not oxidized by exposure to air, and possesses valuable properties. 376. Coal, Peat, Coal-oil and Natural Gas form a group of natural fuels of great use to man. Coal is by far the most important of the natural fuels. Its formation is practically limited to the carboniferous age. (See paragraph 75.) Valuable deposits of coal are found in nearly all parts of the world. The deposits of the United States are prob- ably richer than in any other country. (See paragraph 417.) Peat, an inferior form of fuel, is a carbonaceous deposit that occurs in marshy districts in moist climates. Exten- sive peat-bogs exist in Ireland and in the United States. Coal-oil or petroleum, a valuable form of natural fuel, exists in reservoirs, as already described in paragraph 167. Valuable oil-fields exist in Western Pennsylvania; in Rus- sia; and in other parts of the earth. Natural gas exists in great quantities in the neighbor- hood of the coal-oil districts in the United States and elsewhere. (See paragraph 420.) 377. Clay, Kaolin, Marl, Salt, Sulphur and Graphite——Besides the mineral products already described there is a great variety of others that are employed for various purposes in the arts or sciences. Some of the more important of them are clay, kaolin, marl, salt, sulphur and graphite. Clay and kaolin are extensively employed in the manufacture of bricks, pottery, terra-cotta ware, stone- ware, china and porcelain. Marls are various mixtures of clay and lime, and are employed for fertilizing lands. Clay and marls are extensively found in all parts of the earth. Common salt, or chloride of sodium, is one of the principal saline ingredients in ocean water, and in the waters of inland seas or steppe lakes. It occurs also in vast deposits as rock-salt where it has been derived from the gradual evaporation of saline waters. Various saline or salt springs exist. (See paragraph 166.) Sulphur exists in a native or pure state in volcanic dis- tricts, or combined with various metallic substances as sulphides, it is generally distributed.. It is extensively employed in the manufacture of sulphuric acid. Graphite is a form of carbon extensively employed in lead pencils, and is valuable as a lubricant. In addition to the above mineral products are beds of sand, suitable, when mixed with burnt lime, to form mor- It forms a _ tars and cements; or to form glass, when fused with potash or other basic substances. 378. Building Stones are found in immense deposits near the surface in various parts of the earth. A material to be suitable for building purposes must possess marked strength and tena- city and be able to resist being crushed by the weight placed upon it. It must especially resist disintegration or breaking up under the action of the weather. The more important building stones are granite, gneiss, blue-stone, sandstone, magnesian lime- stone, marble and slate. 379, The Precious Stones or Gems.—Besides the mineral substances already referred to, which with some few exceptions occur in extensive deposits in nearly all parts of the world, there are others which either occur very rarely, or, if common, are but seldom found in fine specimens, free from flaws or other blemishes. These min- erals are called the precious stones or gems, and are highly prized as articles of jewelry. The principal precious stones, or gems, are the diamond, the sapphire, the ruby, the topaz, the emerald, the beryl, the opal, and the garnet. ‘The diamond is a crystallized form of pure carbon. Its value depends on its lustre and color, on its freedom from flaws, and on its size, especially the latter. In the natural state it is generally lustreless and requires to be cut and polished: in order to bring out its fire or lustre. Its value is rated by the carat (approwimately 3 grains). The principal diamond-fields of the world are in South Africa, New South Wales, the Ural Mountains and Brazil. The sapphire is a beautiful blue stone; the best speci- mens are found in Brazil. The ruby has a deep red color; perfect specimens of more than two carats in weight are more valuable than the diamond; the best specimens are found in Siam. The topaz is a yellow stone of various shades; the best specimens are found on Topaz Island in the Red Sea. The emerald is a gem of a beautiful green color; the finest specimens are found in New Granada. The beryl is a stone which occurs in the form of six-sided prisms, usually either blue, green, or yellow, but some- times colorless. The opal, with its changing hues and blending colors, has a strange beauty which defies imita- tion. The garnet occurs in a variety of colors and is quite abundant, being found in all the continents. Other minerals used for ornamentation and jewelry, but less valuable than the above, are turquoise, lapis-lazuli, malachite and quartz. Quartz is a crystal stone of some beauty; amethyst, cat’s-eye, chalcedony, onyx, sardonyx, carnelian, jasper, agate, blood-stone, plasma, and chrysoprase are varieties of quartz of different colors and markings. Pearls are deposits of carbonate of lime and organic matter; they are found within the shells of the pearl oyster and other mollusks; they occur on the tropical coasts of Asia and America, the best specimens are found on the coasts of Ceylon.