188 THE EARTH AS MAN'S ABODE The majority of precious stones are only special varieties of common substances. Aluminum oxide (corundum) is a common mineral; but when it occurs in blue or red colors, we have sapphires and rubies worth from $500 to $5000 a carat. Most of the gems owe their colors to impurities. Thus ordinary quartz (silicon dioxide) is not highly valued, but quartz colored with a little iron or manganese oxide gives us our highly valued purple amethysts. Quartz occurs as rock crystals, rose quartz, and milky quartz. A dense, translucent variety of silicon dioxide is known as chalcedony, some forms of which are agate, sardonyx, carnelian, jasper, onyx, and opal. Of the more than 1500 known minerals, approximately 50 are used as gems. The diamond is a crystalline form of pure carbon which was probably produced from carbon trapped in molten rocks. Most of the precious stones may be synthesized today. With the discovery of this synthesis, superstition surrounding precious stones has decreased, and their values have changed. Now they are valued for what they are rather than for their rarity. Minerals Are Identified by Their Physical and Chemical Properties. It is clear that one may have to examine a great many properties before a sure identification can be made. This is why a knowledge of chemistry is always a useful and sometimes a necessary supplement to microscopic examination and physical tests. Quartz resembles calcite in appearance, but calcite can be scratched with a knife and quartz is too hard to be scratched with a knife. The hardness of a mineral is of considerable aid in its identification. The hardness scale is as follows: 1, talc; 2, gypsum; 3, calcite; 4, fluorite; 5, apatite; 6, orthoclase feldspar; 7, quartz; 8, topaz; 9, corundum; and 10, diamond. The hardness of a mineral is determined by finding which of the above minerals will scratch it and which of these minerals it will scratch. The specific gravity, i.e., its weight relative to the weight of an equal volume of water, is another important physical property. Color, luster, and the manner in which minerals break — either fracturing or forming cleavage planes — are terms frequently used in describing minerals. Coal Was Formed from Liixuriant Land-plant Growths. Today a brownish-black substance known as " peat " is being formed in bogs where plants grow luxuriantly and partially decay as their dead remains fall into the water. Thus peat beds forty feet thick have been formed.
MANY VALUABLE MINERALS 189 Peatlike deposits are then changed into brown Hgnite by moderate pressures resulting from layers of sedimentary rock formed over them. In this process water, marsh gas, oxides of carbon, and other gases escape as the plant compounds are decomposed. Lignite, in turn, loses gases as it is subjected to greater pressure to form bituminous (soft) coal. Soft coal is further decomposed as it is subjected to greater pressure, so that the resulting product is nearly pure, hard, glassy carbon called "anthracite," or hard coal. Some deposits of anthracite have been so compressed and heated that they have been changed into an incombustible form of carbon which is called "graphite." The large coal beds were formed about 250 to 300 million years ago from much more luxuriant vegetation than is found in our present swamps and peat bogs. This vegetation, consisting of huge growths of ferns, horsetails, mosses, and other plants now extinct, must have grown in warm swampy regions. It is possible that the carbon dioxide content of the air was much higher when these plants were growing than it is today. Coal deposits have been found widely scattered, but the most valuable ones are located in North America. No doubt this important natural resource has greatly contributed to the present advanced position of the United States. Coal deposits representing all of the different stages of their formation are found in the United States. Extensive coal deposits in the eastern Ohio and western Pennsylvania fields are bituminous in nature, but farther east the folding of the mountains as they were lifted up changed these deposits to the finest anthracite coal in the world. Still farther east in the Adirondack Mountains, so great were the forces brought to bear that graphite was formed from similar coal deposits of a different age. It is not unlikely that coal beds are being formed even now on a small scale in such places as the Great Dismal Swamp, covering fifteen hundred miles in the southern part of the United States, and in the extensive swamp lands near the equator. It is estimated that it would take about four hundred years or more to produce a layer of coal one foot thick. Some of the individual coal layers in the Mammoth Bed of Pennsylvania arc from fifty to sixty feet thick. Petroleum, Natural Gas, and Asphalt Were Produced from Marine Organisms. Petroleum was used by the ancient Babylonians in their buildings, the Romans used it in their lamps, and the Egyptians embalmed their dead with it. Inasmuch as practically all petroleum deposits have been found in