Most jewelry is fashioned out of precious metals and jewels that are found buried in the Earth, but pearls are found inside a living creature, an oyster. Pearls are the result of a biological process -- the oyster's way of protecting itself from foreign substances.
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 occurrence. Most pearls are produced by oysters in both freshwater and saltwater environments. To understand how pearls are formed in oysters, you must first understand an oyster's basic anatomy.
Oysters are bivalves, which means that its shell is made of two parts, or valves. The shell's valves are held together by an elastic ligament. This ligament is positioned where the valves come together, and usually keeps the valves open so the oyster can eat.
These are the parts of an oyster inside the shell:
Mouth (palps)
Stomach
Heart
Intestines
Gills
Anus
Abductor muscle
Mantle
As the oyster grows in size, its shell must also grow. The mantle is an organ that produces the oyster's shell, using minerals from the oyster's food. The material created by the mantle is called nacre. Nacre lines the inside of the shell.
These mollusks line the inside of their shells with a material called nacre. When a unfamiliar body enters the mollusk, the animal protects itself by depositing layer after layer of smooth nacre around the irritant. This bead of nacre is called a pearl. Most mollusks are capable of producing pearls, but the common sources of commercial pearls are oysters, which produce saltwater pearls, and mussels, which produce freshwater pearls.
The formation of a natural pearl begins when a unfamiliar body enters the
mollusk/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 reaction is to cover up that irritant to protect
itself. It protects itself by depositing layer after layer of smooth nacre around the irritant. This bead of nacre is called 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 objects, which are the most valuable ones. Not all pearls turn out so well. Some pearls form in an uneven shape -- these are called baroque pearls. No two pearls are exactly alike. Round pearls are the rarest and usually the most valuable. The smallest pearls are called seed pearls.
A pearl's surface is covered with tiny lines and craters. A good pearl has a delicate luster. Each layer is translucent and allows some light to pass through to the lower layers. The light reflecting from the various layers combines to create the luster and a slight rainbow effect called iridescence.
The colors of pearls can be black, brown, gray, rose, red, blue, green, purple, yellow, and white. No one knows how pearls make their color, since one oyster may create pearls of many different colors at the same time. The most expensive pearls are white and silvery-white saltwater pearls. Black-lipped oysters from the South Pacific sometimes form black pearls. Many cultured pearls are bleached with peroxide to remove dark spots.
Cultured pearls are created by the same process as natural pearls, but are given a slight nudge by pearl harvesters. To create a cultured pearl, the harvester opens the oyster shell and cuts a small slit in the mantle tissue. Small irritants are then inserted under the mantle. In freshwater cultured pearls, cutting the mantle is enough to induce the nacre secretion that produces a pearl -- an irritant doesn't have to be inserted.
While cultured and natural pearls are considered to be of equal quality, cultured pearls are generally less expensive because they aren't as rare.
