PEARL
Is not a mineral, but the production of a shell-fish. It ranks, however, among the most valuable of the precious stones.
It
is formed by the secretions of a mollusk or shell-fish which forms
lustrous concretions of carbonate of lime, inter-stratified with animal
membrane. Latest research shows that a parasite or foreign substance
within the shell becomes surrounded by a soft, jelly-like material
enclosed in a sack. This gradually hardens, and is later covered by
concentric layers of nacre. Dr. H. Lyster Jameson reports to the
Zoological Society, London, that he found the pearl nucleus to be dead
larva of a distoma or fluke. These spend their early life in the bodies
of fresh-water shell-fish.
Pearl
oysters have shells with a nacreous lining, varying in size from two to
eight inches. The marine or meleagrina shell is square, with rounded
corners and very thick sides. From these are taken the Oriental pearls.
The fresh-water, or unio, is an even, egg-shaped mussel.
The
meleagrina are found in the Indian and Pacific Oceans, and off Lower
California and parts of the coast of Australia, on hard rocks or
sandstone, to which they cling by a fibrous beard. The unio is found in
the brooks and streams of Europe, China, and the United States.
It consists of carbonate of lime and a small proportion of organic matter.
Specific gravity, 2.5 to 2.7; hardness, about 4.
Color, white with various tints, yellow predominating; pink, yellow, grays, bronzes, black, etc.
Symbolizes purity, innocence, June.
