In
other cases pressure against the pearl, or the partial inclusion of
foreign substances, especially of an organic nature which decay before
being entirely covered, are possible causes. The reverse of this also
occurs; grains of sand or other minute particles adhering to the
surface are covered by succeeding layers, thereby producing knobs, more
or less observable according to the lapse of time between their
inclusion and the taking of the pearl from the oyster.
If
undisturbed, the fish will by the deposit of sufficient layers of
nacre, fill the intervals and round the surface again. That this is
done in time is shown by the occurrence of pearls having an even dome
over a nucleus formed by a cluster of small round and irregular pearls
enveloped together. In the process of skinning, or the removal of one
or more of the layers of nacre, it is sometimes found that a depression
has been filled by a thickening of the deposits in the hollow; at other
times extra layers fill the space, and these flaking out with the outer
skin reveal the hidden irregularity which lay beneath the round
surface, thus
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