It
happens occasionally, when skinning a round pearl, that one of these
fillings is uncovered and flakes out, leaving the pearl irregular, as
it was in a former stage of its growth.
Although
pearls naturally grow spherically, many free pearls are more or less
buttoned, that is, have a flat place from which the pearl rises like a
dome, high or low. This happens when the pearl is held during growth by
the fish against the shell with a part of its body intervening.
According to circumstances, the pearl varies in form from slightly
button, to a low dome, rising from a plane at its greatest diameter.
Should a pearl of this description become dislodged, the rounding
action of the mollusk would begin at once to obliterate the plane.
If
undisturbed, the process would result eventually in changing the button
to a round or nearly round pearl, but should the pearl be taken from
the fish before the metamorphosis is completed, a depression, or pit,
would mar its contour. When borers intrude through the shell, the
presentation is at once covered with nacre, and successive deposits are
built up around it resulting in the nacreous wart known
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