9— THE FRESH-WATER PEARLS AND PEARL FISHERIES OF THE 
UNITED STATES. 
By GEORGE F. KUNZ. 
THE ORIGIN, NATURE, AND VALUE OF PEARLS. 
Pearls are lustrous concretions, consisting essentially of carbonate of lime, inter- 
stratified with animal membrane, found in the shells of certain mollusks. They are 
believed to be the result of an abnormal secretory process caused by an irritation of 
the mantle of the mollusk, consequent on accident, disease, or the intrusion into the 
shell of some foreign body, as a grain of sand, an egg of the mollusk itself, or perhaps 
some cercarian parasite. It has also been suggested that an excess of carbonate of 
lime in the water may cause the development of pearls. Accepting the former theory 
as the more probable, it is easy to understand how some foreign body, which the 
mollusk is unable to expel, becomes encysted or covered as by a capsule, and gradually 
thickens, assuming various forms — round, elongated, mallet-shaped, sometimes as 
regular as though turned in a lathe. Mr. Charles L. Tilfany, who has given consider- 
able attention to this subject, suggests that the mollusk continually revolves the 
inclosed particle in its efforts to rid itself of the irritation, or possibly that its formation 
is due to a natural motion, which is accelerated by the intruding body. 
In regard to the formation of pearls, the following general statements may be 
made: Whatever may be the cause or the process of their production, these interior 
concretions may occur in almost any molluscan shells, though they are chiefly confined 
to certain groups, and their color and luster depend upon those of the shell interior 
adjacent to which they are formed. Thus the pink conch of the West Indies yields 
the beautiful rose-colored pearls, while those of the common oyster and clam are dead 
white or dark purple, according to their proximity to the part of the mantle which 
secretes the white or the dark portion of the shell. The true pearly or nacreous 
iridescent interior belongs to only a few families of mollusks, and in these alone can 
pearls proper be formed at all, while in point of fact they are actually obtained only 
from a very few genera. 
The families with iridescent interior layers are the following: Among cephalopods, 
the nautilus and the ammonites, the latter wholly fossil. In both these groups the 
removal of the outer layers of the shell reveals the splendid pearly surface beneath. 
Modern nautilus shells are often “cleaned” with dilute acid to fit them for use as 
ornaments; and frequently this is done partially, elaborate patterns being formed by 
leaving parts of the white middle layers to contrast with the pearly ground. Among 
the fossil ammonites the same effect is produced very often naturally by decay of the 
outer layers, and no artificial pearl work can compare with the richness of color — 
literally “ rainbow- hued” — that is presented by many of these fossils from Jurassic 
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