CATALOGUE OF GEMS AND PRECIOUS STONES. 



77 



PEARL. 



Calcium carbonate is most widely distributed in a number of 

 forms, the varieties depending upon differences in origin, crystalliza- 

 tion and structural condition, presence of impurities, etc. With the 

 exception of pearl and coral, the many forms are used more for 

 decorative purposes than for personal adornment. 



Pearls are concretions, or, more correctly, secretions, consisting 

 essentially of calcium carbonate, found in the shells of certain 

 melius ks. They are the result of an abnormal secretory process 

 caused by an irritation of the mantle of the mollusk, resulting from 



Fig. 8.— Baroque pearls. 



the introduction into the shell of some foreign body, such as a grain 

 of sand. 



Pearls possess a luster peculiar to themselves, which is called 

 pearly or nacreous. This luster may exist on the exterior surface 

 only of the concretion, or the outer surface may be dull and dead 

 in luster while an inner surface may be clear and lustrous. Their 

 specific gravity is 2.5 to 2.7; their hardness 2.5 to 3.5. They may 

 be of any shape, and in some instances of considerable size. In 

 color they range from an opaque white, through pink, yellow, purple, 

 red, green, brown, blue, black, in all shades; in addition the}? - may 

 be iridescent. In general, their color and luster will be that of the 

 interior shell surface adjacent to which they are formed. 



The beauty and value of pearls is dependent upon their color, 

 texture or "skin," transparency or "water," luster, and form, the 



