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The pearl, the only animal gemstone, is the result of the oyster protection against foreign
invaders. The oyster releases a substance into the attacker composed of 90% of aragonite
(CaCO3), a calcareous material, 6% of conqueolina (main component of the outer shell) and 4%
of water. The material is deposited on the invader and crystallized quickly. Normally
each oyster produces only one pearl, which takes about 3 years to mature and that is
not always perfectly spherical (a ratio of 1 to 10,000 animals, that's why it is so valuable).
But Japanese invented a way to cultivate pearls. This video shows a Chinese shell that was
cultivated in order to produce various pearls.