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GENESIS OF PEARLS
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condition;
but from the fact that the process continues after the intruder has
been enveloped and rendered as non-irritant as the natural lining of
the shell, it would appear that the introduction of a foreign element
simply draws upon it the normal impulse of the fish to cover with nacre
anything with which it comes in contact, and that the method of doing
it is similar to the instinctive rolling action of the tongue when some
insoluble globule is put in the mouth, for not only do free pearls grow
spherically, but a nucleus fast to the shell is not covered simply but
it grows to a pearl, round and domelike, as nearly spherical as its
juncture with the shell will permit.
Not
only is the composition of a pearl identical with the lining of the
shell where it is formed, but in a general way its appearance and
characteristics are the same, except that free pearls are sometimes
colored when the nacre of the shell is white.
Button pearls, warts and baroques, grown fast to the shell, are usually like the surrounding nacre in every respect.
Salt-water pearls are characterized by the
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