How Do Oysters Make Pearls?

These are the parts of an oyster inside the shell:

As the oyster grows in size, its shell must also grow. This growth occurs when new material is added to the edges of an oyster's shell. This material is produced by the mantle, which is a thin layer of tissue that lines the inner part of the oyster's shell. The mantle has glands that extract minerals from water and convert them to the building blocks of its hard shell.

The mantle secretes calcium carbonate minerals, along with an organic protein, that together form the shell. Calcium carbonate, which is the same material used to make chalk, makes up 98 percent of the shell; calcium carbonate coats the underlying protein structure to form the shell's hard surface.

The oyster's shell, or exoskeleton, has three different layers. The outermost layer is known as the outer proteinaceous periosteum. The middle layer is a prismatic layer. The innermost layer, known as the nacre layer, lines the inside of the shell.

Incidentally, the nacre layer that lines the inside of the shell also is known as the "pearl layer" because of its iridescent, light-reflective qualities. The nacre layer is sometimes called "mother of pearl" and used to make buttons and other ornamental items.

The same nacre that lines the inside of the shell also forms pearls.

The formation of a natural pearl inside an oyster begins when a foreign substance slips 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 reaction is to cover up that irritant by encapsulating the interloper, thereby protecting itself. The mantle covers the irritant with layers of the same nacre substance that is used to create the shell, and these concentric layers of nacre will eventually form a pearl.

Some oyster species, such as Pinctada mazatlanica are able to secrete three or four layers of nacre a day, but each layer is incredibly thin. Most nacre layers that make up a pearl will be as thin as a thousandth of 1 millimeter (0.03 inches), or one micron. It will typically take an oyster at least 24 months to make a natural pearl that is up to 5 millimeters (0.19 inches) in diameter, which is about the height of 20 stacked playing cards.

It is commonly believed that pearls are formed when a grain of sand enters an oyster; however, this has recently been disputed as a myth. While it is technically possible for a grain of sand to be at the center of a pearl, the oyster species that produce pearls are found on sandy ocean or freshwater floors and have the ability to expel sand and other objects like small pieces of seashells.

The majority of natural pearls are formed in oysters as a response to a parasitic intruder. Parasitic organisms like drill worms will burrow through the hard shell of an oyster and trigger its mantle to secrete a barrier around the biological interloper.

The resulting pearl is a foreign substance covered with layers of nacre.

The size of a natural pearl can range from a scant 0.03 inches (1 millimeter) to an average of 0.27 inches (7 millimeters). Pearls that grow to a size of 0.39 inches (10 millimeters) or more are rare and valuable; in general, the larger a natural pearl, the more valuable it is considered to be.

Most pearls that we see in jewelry stores are nicely rounded objects, and they are the most valuable, particularly if they are natural pearls formed inside an oyster. Both saltwater and freshwater natural pearls have the same value, and are graded based on a variety of factors such as luster, nacre thickness, shape, surface quality, size and color. Pearls 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 indigenous to the South Pacific.

When a natural pearl has an uneven — rather than perfectly round — shape, it is called a baroque pearl. This frequently occurs when the nacre layers encounter resistance during formation, often because the pearl is lodged in muscle tissue within the oyster.

Cultured pearls are created by the same process as natural pearls, but require intervention 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 and the nacre tissue begins forming a pearl. Some cultured pearls are created using a grafting process in which a pearl nucleus is inserted into an oyster, providing the seed for the growth of a new pearl. In some cases, simply cutting the mantle is enough to induce the nacre secretion that produces a pearl and an irritant doesn't have to be inserted.

In general, cultured are far less expensive than natural pearls because they aren't as rare. ­Natural pearls are rare; they are the only jewels in the world created by a living creature. Unlike gems that are formed in soil, natural pearls don't require polishing or other human intervention to enhance their value. While cultured pearls are still "real" pearls, they can be produced en masse to better meet demand.