Pearls

Sep 4 18:49 UTC 1991

 A pearl is the result of an irritating substance, such as a grain of
sand, getting inside the shell of an oyster. The oyster covers the 
irritant with  a milky substance called mother-of-pearl. The pearl is
formed by many layers of tiny mineral crystals.
 The pearl is made up of aragonite (a form of calcium carbonate that is
secreted by the oyster), conchiolin (the the cementing, hard organic
substance of the pearl), and water. The water is contained within the 
conchiolin. The mother-of-pearl forms in thin layers that partly transmit
and partly reflect light. Sometimes a pigmentation gives pearls various
delicate colors, which enhance their value.
 Pearls are among the most valuable jewels in the world. They are sold by
a unit of weight called the pearl grain. There are four pearl grains in a
carat.
 Pearl oysters are fished chiefly in the Persian Gulf; off the coasts of
Ceylon, India, and Burma; among the Polynesian Islands; and along the 
northern and western coasts of Australia. There are also American fisheries
off the coast of Lower California, Panama, Haiti, and Colombia.
 Freshwater pearls are found in the Mississippi River and its branches.
 When oysters are collected, they are spread on the sand and left there
until the animal has died and decayed. Then the shells are washed in sea
water and opened in search of pearls.
 It is possible to force an oyster to form a pearl. The method for making
cultured pearls was devised and patented in 1894 by Kokichi Mikimoto.
 A particle such as a mother-of-pearl bead wrapped in a sac of living
pearl oyster tissue is forced inside the oyster-producing shelled animal.
Oysters treated in this way are placed in cages and lowered into the water.
Each year about .03 inch of pearl will form on the beads, so it takes 
several years to produce cultured pearls of a desirable thickness.

response 2 of 2:

Sep 25 15:34 UTC 1991

Interesting!  A.E. Housman in his lecture "The Name and Nature of 
Poetry" says the same thing.  He says that, for him at least, 
poetry is in the category of "a morbid secretion, like the pearl in 
the oyster."  You're in good company, John.