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PEARL
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Pearl is not a mineral in the strict sense of the word, but has long been associated with gems in thought and use.
Like
amber, jet, and coral, pearls are a product of organic or living
forces, not of inorganic nature. Mollusks, chiefly of the order of
bivalves, are the organisms which produce pearls. They are a product,
however, not of health and normal life, but of disease and abnormal
conditions. This is well known by the pearl-fishers, so that, in
searching for pearls, they pass by the young, well-formed mollusks, to
gather only those appearing old, diseased, and distorted. The formation
of pearls by a mollusk is generally believed to be the result of some
persistent irritation of the mantle. The agent of irritation has been
thought to be a grain of sand, a bit of seaweed, an infusorian, a
parasite, or an egg of the mollusk itself. The origin of the pearl has
been supposed to be due to an effort on the part of the mollusk to
protect itself from such an irritant as one of those above mentioned
by secreting over it a calcareous deposit similar to that of which it
forms its shell.
Some
recent investigations by Dr. H. L. Jameson of London go to show that
many free pearls originate through the entry of a trematode worm into
the epithelium of the mantle of a pearl-bearing mussel. The mussel, in
order to protect itself against the parasite, deposits pearly matter
around it. Even if the parasite leaves the mantle the formation of the
pearl will continue. The life history of this parasite is interesting
in that at different times it lives in three hosts. The first, in the
region where Dr. Jameson studied it, is a so-called "tapestry shell,"
the second the pearl mussel, and the third two members of the duck
family. The eggs of the parasite passing out with the faeces of the
duck enter the body of the tapestry shell, then pass to the mussel, and
when the latter is eaten by the duck, reach the intestine of the
latter. This knowledge makes it seem likely that it will be possible
ere long to infect pearl-bearing mollusks with the parasite in large
quantities, and hence to greatly increase the production of pearls.
The
deposit of pearl has the color and character of the interior of the
shell, or if the color of the shell varies in different portions, that
of the part of the shell which is nearest. Unless the interior of the
shell
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