layers
of nacre and conchiolin. This construction is very distinct in a
formation peculiar to the Abalone, consisting of two nacreous shells
joined perfectly at the edges, the inside walls of, both being covered
with rough black conchiolin.
Peculiarities
in the quality of the nacre sometimes give an appearance of uneven
shape which does not exist in reality. The light falling upon such
pearls produces a knobby effect, as though there were protuberances on
the surface. The texture of others is such that when looked at
squarely from the front they appear pyramidal in form, the rounded apex
pointing toward the observer. Such pearls have a soft, waxy appearance
generally.
Another
common imperfection consists of pits in the surface. These may result
from various causes: in many cases from the dislodgement and rolling of
a pearl which has been flattened during earlier stages by pressure in
one position against the shell. Freed from this hindrance to spherical
growth, the later concentric layers would round over the edge of the
flat spot and thereby leave a pit, or cavity, in the centre.
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