nearly 40.
The delicate colouration of the finest pearls is not due to any kind of
pigment, but to the peculiar " intimate " structure of the nacre
producing colour-effects through interference. Occasionally a dull
pearl, when carefully peeled by mechanical means, will reveal a fine
orient beneath, and be consequently greatly improved in appearance by
the treatment.
Pearls
are secreted by the mantle of the mollusc, and in the same way as that
by which the shell itself is formed. Definite areas of the mantle have
definite functions, secreting, as the case may be, either aragonite, or
calcite or the horn-like substance already named. According to the
position of the pearl in the region of the mantle—a position which is
subject to change—so will be the nature of its successive additional
coats. But it will be asked " How does the pearl, the detached pearl
for example, first come into being ? " Its occurrence, if not rare, is
at least abnormal, and is the outcome of irritation to the mantle
caused by the intrusion of some foreign body. This foreign body is
usually a minute parasitic animal (a Cestode larva), but may be a
grain of sand, or some other solid. The irritation stimulates the
secretion of nacre, and the intruder is sooner or later covered with
layer after layer. The Chinese take advantage of this response to the
irritation caused by the introduction of a foreign body in the case of
a fresh-water mussel (Dipsas plicata). They keep the mussels in
a tank and insert between the shell and the animal rounded bits of
mother-o'-pearl or little metal images of Buddha. In either case the
inserted object becomes slowly coated with nacre and looks like a pearl
: the little figures of Buddha generally become cemented to the shell;
a specimen may be seen in the shell gallery of the Natural History
Museum.
The
value of pearls is increased greatly when a considerable number of
well-matched specimens are got together. But the market value of pearls
depends upon so many factors that, even for a single pearl of what may
be called standard quality, and
