Most jewelry is fashioned out of precious metals and jewels that are found buried in the Earth, but pearls are found inside a living ______, an oyster. Pearls are the result of a biological process — the oyster's way of protecting itself from foreign ______.

Oysters are not the only type of mollusk that can produce pearls. Clams and mussels can also produce pearls, but that is a much rarer __________. Most pearls are produced by oysters in both freshwater and _______ environments.

As the oyster grows in size, its shell must also grow. The mantle is an organ that produces the oyster's shell, using ________ from the oyster's food. The material created by the mantle is called nacre. Nacre lines the inside of the shell.

The formation of a natural pearl begins when a foreign substance _______ into the oyster between the mantle and the shell, which irritates the mantle. It's kind of like the oyster getting a splinter. The oyster's natural ______ is to cover up that irritant to protect itself. The mantle covers the irritant with layers of the same nacre substance that is used to create the shell. This ______ forms a pearl.

So a pearl is a foreign substance covered with layers of nacre. Most pearls that we see in jewelry stores are nicely rounded _______, which are the most valuable ones. Not all pearls turn out so well. Some pearls form in an _______ shape — these are called baroque pearls. Pearls, as you've probably _________, come in a variety of various colors, including white, black, gray, red, blue and green. Most pearls can be found all over the world, but black pearls are___________to the South Pacific.