Why Do Clams Produce Pearls: The Fascinating Natural Process Explained

Clams produce pearls when an irritant gets inside and the animal coats it in layers of nacre, although edible clams rarely make valuable pearls.

How pearls form

An irritant (like a parasite or grain of sand) gets inside the mollusk. It coats the irritant in layers of nacre (mother of pearl) to protect itself.

Which mollusks

Pearl oysters and freshwater mussels produce most pearls.

Clams

Some clams can make pearls, but they are rarely valuable.

Cultured pearls

Farmers insert a bead or tissue to start the process.

Time

A pearl takes 1 to 3 years or more.

Edible

Pearls from edible clams are mostly not gem quality.

Pearl formation in steps

A pearl forms when an irritant, such as a parasite or small particle, lodges inside the mollusk’s mantle tissue. The mollusk coats it with nacre, made of calcium carbonate (aragonite) and the protein conchiolin, layer by layer. Natural pearls take years to form and are rare, with only about 1 in 10,000 wild oysters producing a gem-quality pearl by older estimates.

• Most pearls come from pearl oysters (Pinctada) and freshwater mussels (Unionidae), not edible clams.
• Edible clams can form pearl-like concretions, but they lack the shiny nacre and have little value.
• Cultured pearls start with a bead or tissue graft inserted by technicians, a method developed by Kokichi Mikimoto in Japan in the 1890s.

Growth and size

Akoya pearls grow for about 1 to 2 years, South Sea pearls for 2 to 4 years and freshwater pearls for 1 to 3 years in mussels, which can produce up to 30 pearls at once. The cultured pearl industry produces most pearls on the market today. Clams in a restaurant dish are safe to eat, and a pearl found in an edible oyster should not be bitten, since it can damage teeth.

Pearl formation in steps

Pearls form when an irritant, such as a parasite or bit of debris, gets inside a mollusk. The animal coats it with layers of nacre (aragonite and conchiolin). Not all clams make gem quality pearls. Most commercial pearls come from pearl oysters and freshwater mussels.

Growth and size

Cultured pearls begin with a bead or tissue graft and take 1 to 3 years or longer to grow. Natural pearls are rare, about one per thousand wild oysters.

Nacre and species

Nacre consists of aragonite plates bound by conchiolin proteins, built in layers about 0.5 micrometers thick. A pearl 8 mm across contains thousands of layers. Pearl oysters in the family Pteriidae produce gem quality pearls, as do freshwater mussels of the family Unionidae. Edible clams only rarely form low value pearls.

Cultured pearl industry

Kokichi Mikimoto developed cultured pearls in Japan in the early 1900s by inserting a bead nucleus with mantle tissue. Chinese freshwater farming produces about 1,500 tons of pearls yearly. Grading uses luster, surface, shape, color and size. Natural saltwater pearls can cost thousands of dollars, while cultured pearls range from tens to thousands.

Based on data from the Gemological Institute of America and the Cultured Pearl Association