Pearls - Coloured Pearls Page 272



	264	Pearls.

	occasionally in Attornia cepa are purple, while those in Arca Noce are violet. These, however, are of no commercial value.

	Black Pearls. When we turn to the true Black Pearls, which of late are to be reckoned among the most valued of the jeweller's resources, we must probably seek some other pigment as the colouring agent ; and some authorities have suggested that in this case, the carbonate of lime is coloured with certain compounds of silver, Mexico is the great centre for Black Pearls, and it is said that the oyster-beds yielding such Pearls lie near to the washings from certain silver mines. Hence it has been suggested that some salt of silver exists in the water, and that the presence of this body causes the nacre inside the shell to assume a black colour. Any Pearl that may be formed will necessarily have the same composition as the nacreous deposits lining the shell, and will, consequently, be black instead of white. This conclusion is partly based on the fact that of late years certain fresh water Pearls have been artificially dyed black by means of argentiferous pigments; but the colour of these dyed Pearls is not constant ;