looking
formation will be found, on breaking it, to have a variety of skins in
the interior, some of which are very lustrous, others white and chalky,
like the middle shell of the mollusk. Many of these dead pearls are
formed throughout of this material. Others, perfectly spherical, are
simply successive layers of prism groups like the conchiolin plates of
the shell. Upon cutting these through the centre the skins are shown by
the concentric rings marking their divisions and the prismatic
formation appears as glistening lines radiating from the nucleus to the
surface. Under the microscope these layers, which are thicker than the
nacreous skins of true pearls, appear identical with the epidermis
plates, except that they are concentric instead of flat, and are free
from the coarse, rough, conchiolin deposit which forms the extreme
outer coating of the shells. This deposit is also found, however, in
some pearl formations, as many of the abalone baroques, especially when
they are somewhat flat in shape, are like two pearl blisters joined,
with the shell-building process reversed, the rough, black conchiolin
being inside, and the nacre
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