qualities is sufficient to make a stone highly prized; neither does the possession of all of them insure value. Some beautiful and enduring stones are so rare that they are known only to collectors and have no standard market value. Others fail to catch the popular fancy for reasons not obvious to the layman. While the intrinsic qualities go far in determining the desirability of a stone, it is clear that whim and chance have been no small factors in determining the demand or lack of demand for some stones. As in other minerals, value has both its intrinsic and extrinsic elements.
For the leading precious stones above named, the values are more nearly standard throughout the world than for any other minerals, with the exception of gold and possibly platinum. Highly prized everywhere and easily transported, the price levels show comparatively little variation over the world when allowance is made for exchange and taxes. The valuation of precious stones is a highly specialized art, involving the appraisal not only of intrinsic qualities, but of the appeal which the stone will make to the buying public. In marking a sale price for some exceptional stone not commonly handled in the trade, experts in different parts of the world often reach an almost uncanny uniformity of opinion.
It is estimated that the world stock of precious stones approximates three billion dollars, or a third of the world's monetary gold reserve. Because of small bulk and standard value, this wealth may be easily secreted, carried, and exchanged. When the economic fabric of civilization is disturbed by war or other conditions, precious stones become a medium of transfer and exchange of wealth of no inconsiderable importance.
The beauty of a stone may arise from its color or lack of color, from its translucency or opaqueness, from its high refraction of light, and from the manner of cutting and polishing to bring out these qualities. Hardness and durability are desirable qualities. The diamond is the hardest known mineral and the sapphire, ruby, and emerald rank high in this regard. On the other hand the pearl is soft and fragile and yet highly prized.
Geologic Features
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.