AMONG minerals of economic importance carbon minerals hold the unique position of being at the same time of the most common and the most rare occurrence. As far as external appearance indicates, a piece of common coal and the most brilliant diamond are widely separated; with regard to chemical composition they are closely related. Intermediate between the coal of the stoke furnace and the "brilliant" of the jewelry shop is still another well-known form of carbon, the graphite of the lead pencil. These three substances comprise the far greater part of carbon-containing minerals.

In so far as our mind's picture of a mineral is that of an aggregation of crystals of fairly perfect form our consideration of coal as a mineral is erroneous. We must yield to a broader interpretation of the essential characteristics of a mineral and modify our idea so as to include any homogeneous substance (solid, with the single exception of mercury) of fairly definite chemical composition "occurring in nature but not of apparent organic origin." Organic substances are those that are alive or have lived.

Vegetation is, undoubtedly, the origin of all coal, but often much more than a cursory examination is necessary to prove such origin. In the less altered coals the vegetable origin is readily proved by the actual presence of seeds, plant fibers, and other equally apparent organic remains. A microscopic study is necessary for finding the presence of woody fiber in the more metamorphosed form. The word metamorphose comes from the Greek; meta means after or over; morphe is form. A metamorphosis is a change of form or a forming over.

The history of the discovery of the value of coal as a means of producing heat and of the development of the coal-mining industry covers a comparatively recent period. Coal occurs in such quantities near the surface of the earth's crust and its outcrops are so numerous that it cannot have failed to attract the attention of the most ancient of peoples. Indeed, that coal could be used as a fuel is mentioned by a writer, Theophrastus, who lived 300 years B. C. The ancient Celts of Britain are reputed to have evidenced knowledge of the industrial value of coal. It was not until near the middle of the thirteenth century, however, that coal became so important an economic product as to result in statutes granting to certain places the privilege of mining it. After a long period of trial in England the superiority of coal over other fuels was recognized, and stone coal, as the harder form was commonly known, came into general use. In America bituminous, or soft coal, was mined to a slight extent in the latter half of the eighteenth century. The form now commonly used in house-heating furnaces, anthracite, for a long time baffled the colonists in their efforts to make it burn. The knowledge that an anthracite fire is most effective if not continually poked is said to have been acquired generally by accident.

Europe and the United States to-day produce practically all the coal of the world. In Europe, Great Britain, Germany, France, Austria-Hungary, and Belgium are the main sources of supply. Several important coal areas exist in our own country, notably that of the New England basin, with an area of 500 square miles; the Appalachian district, with an area of 65,000 square miles; the northern area, in Michigan, covering 7,000 square miles; the central area, comprising parts of Illinois, Indiana and Kentucky, and including 48,000 square miles; the scattered western area, with a total of 98,000 square miles; the indefinite Rocky Mountain area, and the Pacific coast region, including parts of California, Oregon, and Washington. Coal mining is yet an undeveloped industry in our territorial possessions. Alaska has an abundant supply of coal, and lesser quantities are found in Cuba and Porto Rico.

Mention has already been made of the two common kinds of coal, bituminous and anthracite. These two kinds mark different stages in the transformation from plant organism to mineral product. As the biologist traces the successive steps in the evolution of an individual of a species from germ to adult, so the geologist unfolds before us the wonderful history of a piece of coal from its first appearance on the earth to the time when it is thrown into our fire grate as fuel. Coal is the metamorphosed product of vegetable growths, changed by atmospheric agencies and the internal forces of the earth acting through a total period of perhaps millions of years. In the remote past, ages before man had appeared on the earth, the atmosphere or our globe was highly charged with carbon gases. Vegetation flourished in luxuriance. Great swamps were common. The ocean alternately covered and receded from verdure-clothed land areas. Ponds were transformed to morasses and swamps. In the swamps thus formed, the accumulated sediment of centuries upon centuries covered alternate layers of decayed plant organisms, until finally beds of peat were formed. Great masses above pressed on those underneath; the internal heat of the earth reached up and transformed the densely packed masses of peat until the beds became hard and brown, the product of the partial metamorphism being what we know as lignite, or brown coal. With the continued action of the forces of metamorphism, the lignite turned still darker, and as more gases were driven off, became heavier, until the bituminous stage was reached, which, in turn, was succeeded by the anthracite stage.

Graphite, or black lead, is a mineral containing not more than five per cent of impurities, and is generally supposed to have originated as did mineral coal, and to represent a still more advanced stage of development. It occurs in various localities both in the vicinity of coal measures and far removed from them. The chief part of the world's supply comes from Ceylon, though Germany and the United States produce quantities of graphite of excellent quality. In the Laurentian rocks of Canada, and of course with as ancient origin, extensive deposits are found. This presence of graphite in strata in which as yet no certain traces of organic life have been found has led some to believe that this form of carbon mineral may have another than organic origin.

Various uses are served by graphite. The chemist finds it of great value in making his crucibles; the engineer uses it, finely powdered, as a lubricant; the housekeeper polishes stoves with it; the electrician uses it in his arc lights; all civilized nations use it in the lead of lead pencils. The stem, grapho (to write), on which so many of our words, as geography, telegraph, graphophone, etc., are formed, suggests also the origin of the name, graphite. The finest quality lead pencils are those made from graphite occurring in a state sufficiently pure to allow the cutting and grinding of pieces to the size needed. In the case of the medium and poorer grade pencils, the graphite has first been finely powdered and then pressed into the requisite shape and size.

The purest form of carbon found in nature is the diamond. The rare occurrence of diamonds indicates that the essential conditions in nature for causing the transformation of some less pure form of carbon into diamond are seldom present. While diamonds have actually been produced in the laboratory by far-seeing and indefatigable chemists, yet the cost of such products is so great as to preclude the possibility of the most precious of gems becoming at all common. The diamond is the hardest of all known substances, and will scratch any other mineral across which it may be drawn.

Three localities have successively furnished the main part of the world's stock of diamonds. A century and a half ago, practically all the diamonds came from India, where at one time 60,000 persons were employed in diamond digging. Toward the middle of the eighteenth century, when the diamondiferous districts of India were becoming exhausted, the discovery of the precious gem in Brazilian deposits was made. At present, the supply of diamonds from Brazil has much diminished, and the diamond fields of South Africa, where is located the famous Kimberley mine, produce the larger part of the world's output of diamonds.

Among famous diamonds of the world should be mentioned the Koh-i-noor of the British crown, which, Hindu legend relates, was worn five thousand years ago by one of their national heroes. The largest known diamond, weighing three hundred sixty-seven carats, was found in Borneo, and is now owned by the Rajah of Matan.