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I found this hilarious when I was reading this book. Pearls are primary made of Calcium Carbonate (CaCO3) and conchiolin. And the interesting fact is that certain mollusks form a pearl sac to protect themselves from a potentially threatening irritant such as a parasite inside its shell, or an attack from outside, injuring the mantle tissue. It uses the pearl sac to seal off the irritation.
|A black pearl and a shell of the black-lipped pearl oyster.|
The iridescent colors originate from nacre layers.