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THE PEARL
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seldom
iridescent. Occasionally a pearl of that character is found, but it is
generally from a fresh water mussel, and the nacreous plates are of
unusual tenuity.
Although
the pearl like the lining of the mollusk's shell is composed of
carbonate of lime in series of thin waves lapping each other, each
series constituting a plate or separable layer, there is a distinct
difference in construction.
Whereas
the lining is a series of horizontal layers, the pearl is made up of
concentric layers, each addition enveloping those preceding it. These
skins however are not always absolutely distinct and separate. Instead
of being like a succession of globular skins, each completely covered
by its successor, the growth is often spiral and the construction is as
if the nucleus had been rolled one, two, or three complete revolutions
in a continuous plate of nacre, and the spiral envelope then finally
merged into another plate and the process repeated. That which to a
casual glance, therefore, appears to be six rings of nacre in a
sectional cut, is in reality, several spirals of two or three turns
each.
148
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