How do oysters produce pearls?

Pearls are smooth crystalline calcium carbonate and can form in many species of mollusk. However, the large spherical jewels are formed in "pearl oysters" (pearl mollusks of the family Pteriidae), which are a different family from the edible oyster. There are two methods by which pearl-producing mollusks produce pearls. Neither of these methods involve a grain of sand, although this is a common misconception.

Natural Pearls = No Human Intervention

Natural pearls are produced when the shell of a mollusk is invaded by an intruder, most often a boring parasite. The intruder makes its way into the mantle tissue of the mollusk. This tissue has special nacre-secreting cells known as epithelial cells. These cells form a sac around the intruder (pearl sac) and begin depositing a calcium carbonate substance known as nacre. Nacre is the building block of pearls.

Cultured Pearls = Human Intervention

Cultured pearls are produced in both saltwater mollusks and freshwater mussels. The process involves coaxing open the shell, and in the case of saltwater mollusks, making a small incision on the reproductive organ and inserting into this incision a rounded bead made of freshwater mussel shell and a small piece of mantle tissue from a donor mollusk. The same process of natural pearl production then takes place.

In the case of cultured freshwater pearl production, incisions are made to the mantle organ of the mussel, up to 16 on either side of the valve, and small pieces of donor mantle tissue are inserted. This is followed by the same process of natural pearl production.

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Although clams can produce "pearls", they are non-nacreous and have little commercial value. Mollusks from the family Bivalvia are the producers of pearls. There are 8000 different species of which only approximately 20 will consistently produce pearls.

Clams that do produce pearls likely produce them in a similar fashion as pearl-producing mollusks, by the invasion of a parasitic worm or other organic intruder - NOT a grain of sand. That is an urban legend.

The edible oyster is not a pearl producer and the term "pearl oyster" is a misnomer. The correct scientific term would be pearl mollusk.