Coloured Pearls. 261 



fancies, however, we may refer to the opinion so 

 often expressed and still entertained in some quarters, 

 that the black colour of a Pearl is traceable to some 

 disease in the Pearl-bearing mollusc. 



Although the origin of the colour is in the 

 deepest degree obscure, it seems probable that it 

 is in some cases due to the presence of certain pig- 

 ments in the medium in which the molluscs live. 

 The subject of the colouring matter of the nacre 

 in the shells of the genus Unio, afforded matter for 

 an interesting discussion at the meeting of the 

 Academy of Natural Sciences of Philadelphia, on 

 March 20, i860. If we know the nature of the 

 pigment which colours the nacreous lining of the 

 shell, we may safely conclude that we know also 

 the character of the colouring matter in the tinted 

 Pearl ; inasmuch as a Pearl is of precisely the same 

 nature as the nacre of its shell Here we refer not 

 to the pearly hue of a nacreous shell, which, as ex- 

 plained in an early chapter (p. 87), is a purely 

 optical phenomenon, but to the substantive colour 

 of the carbonate of lime which constitutes both 

 the nacre and the Pearl, and which colour is, no 

 doubt, due to the presence of some material pig- 

 ment. The late Dr. James Lewis, of Mohawk, 

 New York, suggested that the colour of many fresh- 

 water shells might be caused by certain salts of 



