in
a very small decree that " orient " or iridescent sheen which
constit'ates the peculiar charm of this gem ; but some specimens of
great beauty have been found from time to time. A pearl of particular
purity from the river Conwav, North Wales, was presented to the Queen
of Charles II. by Sir R.
Wynne, and is now in the Royal Crown. The author of this handbook has
seen a few fine pearls taken from mussels in the river Irt in
Cumberland. The somewhat clouded " orient " of the majority of British
pearls accords with dead or malt gold and with many deep-coloured
stones.
Pearls
are sometimes found having a decided tinge of colour ; rose-coloured,
salmon-pink, pale blue, russet-brown, olive-brown and black pearls are
highly esteemed ; dull and muddy hues are less appreciated, and so also
are extremely small pearls, which, indeed, are by far the most
abundant. Pearls may be dyed easily, and are liable to become
discoloured by wear. Pink coral, cut into suitable forms, is often made
to simulate pink pearls, but its texture is entirely different, and may
readily be recognised with the aid of a pocket lens. Black h;ematite,
one of the chief ores of iron, makes, when not too highly polished, a
passable imitation of a black pearl ; but nothing is easier than to
detect the substitution, for haematite is more than twice as heavy as
the pearl.
The
substance of the pearl is identical, or practically identical, with the
nacreous material, the mother-o'-pearl, which lines the interior of the
shell. It consists of that form of calcium carbonate which is known as
aragonite and is rather harder and heavier than calcite, the other and
commoner form. The aragonite in a perfect pearl is arranged in regular
concentric layers, like the coats of an onion, and is always associated
with a small quantity of an organic substance allied to horn. In some
pearls the horn-like body occurs in larger proportion and may even
constitute one or more distinct layers. And occasionally layers of the
commoner form of calcium carbonate, that is calcite, occur in
pearls—such layers are quite dull. The specific gravity of pearls is
about 267. their hardness
