264 Pearls. 



occasionally in Anomia cepa are purple, while those 

 in Area Noes 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 ; 



