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. 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 transpor