from Mount Vesuvius, Siberia, Germany, Norway and California.
26. Zoisite belongs
to the epidote group, orthorhombic. Colour: grey, greenish or red. The
manganese variety is known as thulite, and is used for ornamental
objects or out cabochon. It occurs in Norway and in Piedmont, Italy.
Chapter XIXVI
GEM MATERIALS OF ORGANIC ORIGIN Pearls
PEARLS,
though an animal product, are held in great esteem by jewellers and
lovers of gems in all parts of the world. They were known to the
ancients, and held the highest place among gems, since the beauty of
the pearl is entirely due to nature. The Hindus ascribed its origin to
God Vishnu. Pearls were also held in great esteem by the Persians, and
were known even in 2300 B.C. In ancient times, pearls were also paid as
tributes. Pearls were also considered as emblems of purity, beauty and
nobility and also to have possessed medicinal effects.
Pearls
are found in the shells of oysters and mussels. These animals withdraw
calcium carbonate from the sea-water and use it for building their
shells, which are composed of organic material conchiolin and calcium
carbonate in its two crystalline forms calcite and aragonite. The
outside of the shell is conchiolin and the inner portion is composed of
two layers—the outer of these consist of calcite and conchiolin and the
inner of aragonite and conchiolin. The inner layer furnishes mother of
pearl, etc.
Origin of pearl.—When
a grain of sand or any other foreign object enters the shell of a
mollusc, irritation is caused, which results in the secretion about the
offending particle or parasite, layer by layer and then the pearl is
built up. The knowledge of this phenomenon by man has
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