Unlike gemstones produced deep inside the Earth, pearls are created by living creatures called mollusks. Mollusks commonly have a soft, unsegmented body and a hard exterior shell, such as a clam or snail has. These animals live in marine and freshwater habitats as well as on land. The evolutionary history of this group extends back some 530 million years, with approximately 100,000 species of mollusks alive today.

Any mollusk that produces a shell can produce a pearl. Nevertheless, naturally occurring pearls are rare, found in perhaps one of every 10,000 animals. The cultured pearl industry, which has flourished since the early 20th century, has developed techniques to greatly improve these odds. Indeed, more pearls are produced now than at any time in human history.
Composition

Contrary to popular belief, pearls hardly ever result from the intrusion of a grain of sand into an oyster's shell. Instead, a pearl forms when an irritant such as a wayward food particle becomes trapped in the mollusk. The animal senses the object and coats it with layers of aragonite ("ah-RAG-uh-nite") and conchiolin ( "KON-kee-uh-lin"). These two materials are the same substances the animal uses to build its shell.
Structure

In most pearls, the mineral aragonite is arranged in sheets of flat, six-sided crystals. Between each sheet, the mollusk secretes a very thin layer of the membrane-forming protein conchiolin. This composite material is called nacre ("NAY-ker") or mother-of-pearl. The crystalline structure of nacre reflects light in a unique way, giving so-called nacreous pearls their high luster. In contrast, some pearls are not nacreous and instead have a low-luster, porcelainlike surface. The needlelike crystals of aragonite in these pearls are arranged perpendicularly or at an angle to the surface of the pearl.
The Rough Exterior

Because a pearl is the product of a biological process, its surface often shows minor imperfections. Furthermore, when a mollusk secretes the microscopic layers that make up a pearl, each layer does not always encircle the entire pearl. These uneven layers create additional irregularities on the surface. As a result, it is easy to distinguish a real pearl from an artificial one by rubbing it gently across your teeth: a real pearl will feel gritty and an artificial pearl will feel smooth and slippery.
A Perfect Pearl?

Pearls come in a wide variety of sizes and shapes. And although most people think of pearls as round, the truth is that irregularly shaped pearls are more common in nature, while perfectly spherical pearls are extremely rare. A pearl's size and shape depend on the species of mollusk that produced the pearl, how long it took to form, the size and shape of the nucleus and where the pearl formed inside the animal. Pearl farmers increase their chances of obtaining large, round pearls by using large, perfectly spherical nuclei. Even so, there are no guarantees. A pearl's size and shape reflect such variables as the temperature and chemistry of the water, as well as the health of the mollusk.