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Pearls
Nature Bulletin No. 614-A October 23, 1976
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation
PEARLS
About ten years ago, a large valuable pearl was found in a mussel taken
from the Wabash River. In years past, news of such finds often started
outbreaks of "pearl fever. " Then, people flocked to rivers and creeks
where they waded, dived, raked and dredged for mussels. 'These were
broken open, searched for that dreamed of pearl and tossed aside. Some
were successful. For instance, in the years 1889 to 1897 a quarter of a
million dollars worth of pearls were sold from the Mackinaw River, a
small stream in central Illinois. In recent decades, however, these slow-
growing mollusks have become scarce due to exploitation and
increasing pollution. Now, the discovery of a freshwater pearl is a rare
event.
Clams, oysters, mussels and many other mollusks with limy shells are
known to produce pearls. but very few kinds yield gem pearls of
jeweler's quality. The pearl is an abnormal growth of mother-of-pearl,
or nacre, imbedded in the soft bodies of these shellfish. It is built up,
layer upon layer, in the same way as nacre is added to the lining of the
growing shell and always has the same color and luster. For example,
over the country, hundreds of good-sized pearls are found each year in
the oysters we eat. Unfortunately these have no commercial value
regardless of whether they have been cooked or not because they are
dull opaque white or purple like the shell of the parent oyster. In recent
times almost all pearls of gem quality come from the oriental pearl
oyster which has a bright shimmering translucent nacre.
A pearl starts growing when some irritating foreign substance such as a
sand grain, bit of mud, parasite or other object becomes lodged in the
shell-producing gland called the mantle. Pearls formed in the soft flesh
where nacre can be added on all sides are most likely to be spherical
and the most highly prized. By far the great majority are flattened or
variously distorted and have little value. Size, color, luster and freedom
from flaws are other essential qualities. Unlike other gems, such as
diamonds, pearls have an average life of only about 50 years. In time
the small amount of water in a pearl's make-up is lost and its surface
cracks. Because they are mostly lime, necklaces which are worn often
are injured by the acid secretions of the human skin.
Centuries ago the Chinese learned that beads or tiny figures of Buddha
slipped between the soft mantle and the shell of a living clam became
coated with a blister of mother-of-pearl and could be sold as a curio or
religious object. With this as a hint a Japanese pearl hunter named
Mikimoto began to cut mother-of-pearl beads and insert them between
the mantle and shell of pearl oysters and return them to an ocean bay
near his home. Many of these grew in size and took on the sheen and
beauty of true pearls but, because they were in contact with the mantle
on only one side, they became half-pearls with little value as jewelry.
After years of experimenting, Mikimoto finally learned how to grow
spherical gem pearls and founded the great cultured pearl industry of
Japan. Briefly, this is the method. A mother-of-pearl bead is wrapped in
a piece of the living mantle of one pearl oyster and is skillfully
transplanted into the flesh of another. Women divers care for the
oysters by the tens of thousands in special floating cages. After three or
more years, a small percentage of them yield pearls no less beautiful
than the best "wild" pearls.
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Update: June 2012
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