the
objects are found to be coated with a pearly substance. Pearls so
formed, however, are comparatively dark and lusterless, and have by no
means the value of those of wholly natural origin.
Pearls
vary in size from those of microscopic dimensions to those as large as
a pigeon's egg. The latter are, of course, very rare.
The
Shah of Persia is said to possess the largest pearl known, it being
about one and one-third inches in length in one direction and one inch
in another. A pearl in the Austrian crown weighs 300 carats, and one in
the South Kensington Museum weighs 455 carats. The small pearls used in
jewelry are known as seed, dust, or sand pearls.
Pearls
are chiefly white in color, but many are tinted yellow or pink; some
are gray, green, or purple, and many other colors occur. Black pearls
are obtained in certain fisheries. The pearls from the Unios of North
America are of almost every shade.
Pearls
of pure white color, if of the proper luster, are those most highly
prized in Europe and America, although a slight pinkish tinge does not
injure the value. They must, however, have the transparency of the true
pearl and not be "chalky." In China and India pearls of yellow color
are preferred.
The
hardness of pearl is 4, and its specific gravity 2.65-2.68. It is thus
like that of shell or " mother of pearl," which might be expected from
the fact that both are of the same chemical composition—carbonate of
lime. Owing to their low hardness pearls are easily scratched, and on
account of their composition are attacked by acids. They thus
deterioÂrate with age, losing their polish and luster and often
becoming black and unsightly. No way of positively restoring the luster
of pearls is known, although occasionally the outer marred coating can
be removed by those skilled in the art, and a lustrous surface be found
beneath.
The
Ceylonese are accustomed to feed pearls which have become dull to
chickens. After the pearl has remained in the crop of the bird a few
hours the fowl is killed and the pearl removed. The movement and
friction to which the pearl has been subjected in the bird's crop are
usually found to have restored its luster to some extent.
To
preserve pearls as long as possible they should be wiped with a clean
linen cloth each time after being worn, and be kept in a dust-tight box
carefully wrapped in linen. Hot or boiling water injures and in time
destroys their luster, and many valuable pearls have been ruined
because the mollusk which contained them was boiled before opening.
The mollusks which yield pearls are many, and pearl-fishing is an industry carried on in many parts of the globe.
The pearl mollusk or pearl oyster, par excellence, is that known by
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