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How Pearls Form
Natural pearls take years or decades to grow to an economically useful size. Once a parasite or bit of decaying organic matter gets through a mollusk's shell, the animal secretes a fluid that encapsulates the irritant. The fluid contains cells that produce shell material. Over time, these cells surround the irritant with a sphere of smooth shell lining. Sometimes, the pearl forms as part of the shell; jewelers term this a blister pearl. Perfectly spherical pearls unattached to shells fetch the highest prices.
Calcareous versus Nacreous Pearls
Nacre, the substance that gives pearls their iridescent luster, contains calcium carbonate and an organic chemical called conchiolin. Not every species of clam forms nacre, although all species can form pearls. In the case of some clam species, pearls are calcareous -- opaque and shiny, but not iridescent -- rather than nacreous. Only a clam that has an iridescent shell lining can produce an iridescent nacreous pearl. Because they lack the pearly luster that makes them valuable, calcareous pearls are not considered gems.
Cultured Pearls
Ancient pearl collecting relied on haphazard means of producing pearls. Thousands of clams or oysters had to be opened to find even a single valuable pearl. Culturing pearls means seeding a clam or oyster with a bead upon which the organism builds layers of nacre. Cultured pearls take significantly less time to form than natural ones. Clam pearl cultures usually start with a layer of clam shell as the nucleus.
Pearl Colors
The mantle of a mollusk's shell gives an indication of what pearl colors it can produce. The black-lipped oyster takes its name from the rim of dark nacre along its edge; this creature produces rare and valuable black pearls. In clams, pearls are typically white, pale pink or golden. Some varieties of clams produce purple or lavender pearls, but most of these have no nacre, so they resemble ceramic beads more than iridescent pearls. Pearls take well to dyes. Many colorful strands of pearls have likely been dyed instead of coming from the shell in such hues.
Pearls on the Dinner Plate
While it's possible to find a pearl in a plate of steamed clams, it's rare it will have any economic value. The species of clams that people typically eat are not the same that produce nacreous pearls. Pearls found in seafood dinners are more likely to be of the calcareous variety.