The principal precious stones above named are of simple composition. Diamond is made of carbon; the pearl is calcium carbonate; ruby and sapphire are aluminum oxide—varieties of the mineral corundum; the emerald is silica and alumina, with a minor amount of beryllia. Minute percentages of chromite, iron, manganese, and other substances are often responsible for the colors in these stones. Carbon also constitutes graphite and is the principal element in coal. Lime carbonate is the principal constituent of limestone and marble. Alumina is the principal constituent of bauxite, the ore of aluminum, and of the natural abrasives, emery and corundum. Silica, the substance of common quartz, also constitutes gem quartz, amethyst, opal, agate, onyx, etc.
Most of the world's diamonds come from the Kimberley and Transvaal fields of South Africa, where they are found in a much decomposed volcanic rock called "blue ground." This is a rock of dull, greasy appearance consisting largely of serpentine. It was originally peridotite, occurring in necks or plugs of old volcanoes penetrating carbonaceous sediments. When the rock is mined and spread at the surface, it decomposes in the course of six months or a year, allowing it to be washed and mechanically sorted for its diamond content. The amount of ground treated in one of the large mines is about equal to that handled in operating the huge porphyry copper deposit of Bingham, Utah; the annual production of diamonds from the same mine could be carried in a large suit-case.
The diamonds were clearly formed at high temperatures and pressures within the igneous rocks. It has been suggested that the igneous magma may have secured the carbon by the melting of carbonaceous sediments through which it penetrated, but proof of this is difficult to obtain. Artificial diamonds of small size have been made in the electric furnace under high-pressure conditions not unlike those assumed to have been present in nature.
Weathering and transportation of rocks containing diamonds have resulted in the development of diamond-bearing placers. The South African diamonds were first found in stream placers, leading to a search for their source and its ultimate discovery under a blanket of soil which completely covered the parent rock. The proportion of diamonds now mined from placers is very small.
The diamonds of Brazil come from placer deposits. This is the principal source of the black diamond so largely used in diamond-drilling.
The United States produces no diamonds on a commercial scale. Small diamonds have been found in peridotite masses in Pike County, Arkansas, but these are of very little commercial value. A few diamonds have been found in the glacial drift of Wisconsin and adjacent states, indicating a possible diamond-bearing source somewhere to the north which has not yet been located (p. 317).
Pearls are concretions of lime carbonate of organic origin, and are found in the shells of certain species of molluscs. Their color or luster is given by organic material or by the interior shell surface against which the pearl is formed. The principal supply comes from the Indian and Pacific Oceans, but some are found in the fresh water mussels of North America, in the Caribbean, and on the western coast of Mexico and Central America.
From the beginning of history the principal source of rubies has been upper Burma, where the stones are found in limestone or marble near the contact with igneous rocks, associated with high-temperature minerals. The weathering of the rock has developed placers from which most of the rubies are recovered. Siam is also an important producer. In the United States rubies have been found in pegmatites in North Carolina, but these gems are of little commercial importance.
Sapphires are of the same composition as rubies and are found in much the same localities. Most of the sapphires of the best quality come from Siam, where they are found in sandy clay of placer origin. In the United States sapphires are recovered from alluvial deposits along the Missouri River near Helena, Montana, where they are supposed to have been derived from dikes of andesite rocks. In Fergus County, Montana, they are mined from decomposed dikes of lamprophyre (a basic igneous rock). In North Carolina sapphire has been found in pegmatite dikes.
The principal source of fine emeralds is in the Andes in Colombia. Their occurrence here is in calcite veins in a bituminous limestone, but little seems to be known of their origin. The only other emerald locality of commercial importance is in the Ural Mountains of Siberia. Emeralds have been found in pegmatite dikes in North Carolina and New England, but the production is insignificant.