MOTHER-OF-PEARL AND BLISTER PEARLS 139 



sponge or burrowing worm which has begun to penetrate the outer layers of 

 the shell, or the intrusion and presence of grains of sand or similar material be- 

 tween the mantle and the shell. To allay the irritation the mollusk deposits nacre- 

 ous material upon the spot. This accumulation forms a little mound which con- 

 forms to the shape of the irritating object and closely resembles a segment of a 

 large pearl. In some cases it results from the mollusk covering a choice pearl which 

 has become loosened from the soft tissues and adheres to the shell and which may 

 be secured by breaking the blister. 



The blister pearls found on the abalone shell are formed chiefly in defense of 

 the invading, boring mollusk Pholadidea parva. They occur mostly in the red 

 abalone, with only one blister pearl in about 1000 shells of the green or black 

 species. A crab, which infests the abalone at certain seasons, may be the cause 

 of such formations. Frequently, the blister pearls are found over sea urchin 

 spines, chiton, or razor clam shells, pebbles, and other foreign bodies retained 

 beneath the surface of the mantle. Sometimes a diseased visceral lump is cut o£F 

 and covered by nacre, making a huge blister pearl. 



Although pearl bhsters appear empty, they are usually filled with some fluid 

 resulting from animal and vegetable decomposition. When a pearl shell shows 

 any protuberance on the surface, the peeler will cut or scrape away a portion of 

 the decaying shell behind the spot. If he finds it to be perfect, and not the hole 

 of a boring insect, this proves that the protuberance has been forced out from 

 within. The pearl is then removed by breaking the shell or by cutting around the 

 protuberance very near its edge. Layer after layer of the covering mass is re- 

 moved with the greatest possible care. 



Closely resembling the "blister pearls" are those known as bouton pearls. In 

 this case the pearl-forming sac becomes ruptured and a pearl growing in the 

 tissue of the mollusk, very near the nacreous lining of the shell, gradually becomes 

 attached to the shell. The growth of the under side is arrested, while on the 

 upper, exposed surface further coatings of nacre are deposited. This results in 

 the formation of a "button"; the constantly growing shell finally encloses the 

 pearl, which in some cases is forced backward until it is pushed through and 

 out of the shell. From this source are derived great numbers of half and quarter 

 pearls, which are usually faulty and of indifferent luster. 



While the chemical composition of these pearls is the same as that of those 

 regularly found in the mollusk, the physical properties differ to a certain extent. 

 The nacreous layers in the true blister pearls lack the regular concentric disposi- 

 tion characteristic of spherical pearls, and the same holds true for the outer layers 

 of the encrusted pearls. 



Utilization 



In America the only important source of blister pearls is the abalone shells. 



Bhster pearls are utilized in jewelry in many ways. Beautiful pins and brooches 

 are made from them. The finest are often mounted in rings. Blister pearl cuff links 

 are much in vogue. 



These pearls have been worked into fantastic figures using pearls and other 

 jewels for parts of the designs. Some of the most beautiful designs are to be seen 

 in the famous Green Vaults' Collection in Dresden. One of the most curious is the 

 figure of a dwarf formed of a baroque pearl, studded with small diamonds. 



