Oysters are useful for food and the production of
pearls they give. Pearls begin to form by a parasite
or a grain of sand getting into the mantle of the oyster.
Its presence causes an irritation for the oyster.
The oyster secretes its mother-of-pearl-forming-substance
around the object which stops the irritation. Over
time many layers of the substance are made and
the pearl is formed. The Japanese discovered a way
to encourage the oysters to make pearls artificially.
It takes three to four years for a pearl to form.
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