It
is also noticeable that whereas the wave edges, with all their
eccentricities, trend generÂally in one direction in the shell nacre,
in the pearl, the lines twist and curl with a concentric tendency, as
though the waves had been laid on by turning or rolling the pearl in
the material of which it is composed.
A
white pearl on being cut in half shows a number of faint dark rings one
within the other, from the surface to the nucleus in the centre;
usually these rings occur at almost regular intervals. Upon close
examination under the microscope, it will be seen that the inner part
of these intervals is white, and that the color gradually changes to a
yellowish tint which deepens until it culminates in that which appears
as a dark line against the succeeding outer formation, the material of
which is also white in the beginning. Although this change of color is
very-slight, a section between two rings will often show three distinct
bands; the inner white, the centre one faintly yellow and the outer one
of a deeper tint. In some cases the dark concentric rings succeed each
other very closely, in which case no abrupt changes of
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