58 PEARLS [CH. 



substance occurs too, both in the very centre as a 

 nucleus, and also in concentric layers alternating 

 with the prismatic substance (fig. 6). 



The prismatic substance resembles closely that 

 same layer in the shell, in the manner in which it is 

 deposited, but the columns or prisms are radially 

 arranged in the pearl so that they are at right-angles 

 to the concentric laminae. 



INIost pearls consist of a nucleus, to which reference 

 will be made later, and more than one, perhaps all, 

 of the different varieties of shell substance. The 

 most important of these is of course the nacre. 



The figure below is that of a nacreous pearl com- 

 posed of two pearls which have fused together. The 

 nuclei consist* of periostracal substance and also 

 hypostracal substance. Round these nuclei there 

 are concentric laminae of nacre with two rings of 

 periostracum. These concentric layers of peri- 

 ostracum may spoil the appearance of a pearl, com- 

 posed otherwise almost entirely of nacre. In the 

 same way, layers of prismatic substance may interfere 

 with the continuous deposition of the nacre. 



Owing to the deposition of the layers in con- 

 centric form, it is possible for a clever jeweller to 

 strip ofi* the laminae, just as the "skins" can be 

 removed from an onion, though of course the laminae 

 of nacre are never continuous right round a pearl. 



This process of skinning the pearl consists in 



