cramped,
they are held against the unyielding shell and grow flat on one side.
These are the button pearls. Others either from an irregular rolling,
or unequal action of the mollusk's mantle, become imperfectly round.
Sometimes foreign particles attach themselves to a growing pearl and
becoming enveloped with it in future layers, make an uneven surface.
Not
infrequently two round pearls grow side by side until they touch, and
together are enveloped by succeeding deposits; a twinned pearl is the
result. For some reason, drop and pear shape pearls are seldom
imperfect in shape. They may not be ideal but the form is usually good
and the contour even and regular. This would imply that the simple
rolling motion by the fish is more regular than the more complicated
movements necessary to form a sphere.
Imperfections
in the texture and luster of the skin are said to be due to the
movement of the growing pearl among the zones of the mollusk's mantle
supplying the varied material for the epidermis, middle shell, and
lining. The difficulties confronting this theory are explained in the
chapter on the "Genesis of Pearls."
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