PEARLS AND IMITATION PEARLS 97 



structure. Pearls are more or less spherical forma- 

 tions constructed by the moUusk in the same way 

 and from the same secretions with which it lines the 

 shell, misdirected by abnormal conditions. Ordina- 

 rily the mollusk secretes nacre merely for the 

 construction of the shell ; but if some foreign object 

 gets within the shell it is covered with a deposit of 

 nacre, and thus a minute pearl is formed. Probably 

 the oyster covers the object with nacre as a means 

 of self-protection, alleviating the irritation caused 

 by foreign matter within the shell. The mollusk de- 

 posits layer after layer of nacre on the nucleus, and 

 thus the pearl grows larger and larger. Until recently 

 the nuclei of pearls were thought to be grains of 

 sand, but careful research by modern scientists has 

 shown that there are several conditions w^hich may 

 cause the formation of a pearl. The irritating 

 granule may result from the decomposition of an 

 organism or may be generated in the mantle of the 

 pearl-oyster as material for the outer shell, which, 

 failing to be utilized in this way, becomes a source 

 of irritation to the tender mantle and therefore is 

 provided with a nacreous coating. 



While the structure and composition of a pearl 

 are similar to those of the shell of the mollusk in 

 which it was formed, yet there is a distinct difference. 

 The mother-of-pearl lining of a shell is composed 

 of a series of horizontal layers, or skins, as they are 

 called. Moreover in the shell nacre the wave edges 

 tend generally in one direction, whereas in the pearl 

 the lines twist and curl with a concentric tendency. 



Chemically the composition of pearls is identical 



