in a very small degree that "orient" or iridescent sheen which constitutes 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 Conway, 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 hæmatite, 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 hæmatite 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 2·67, their hardness
