If they can't get rid of it, mussels try to encapsulate a foreign structure with material that immobilizes a potentially dangerous invader. And if the mussel is lucky enough to have some of the shell-forming mantle tissue around the invader, the cells of this tissue will produce "mother-of-pearl" to form layers around it and eventually, a pearl is formed. Mother-of-pearl is also used in the production of the shell, and it is the component that makes some molluscan shells so shiny and attractive to human observers.
In fact, every mollusc that produces a shell can also produce pearls. But it would be too nice if all these numerous species would produce marvelous pearls when attacked by a parasite. The probability to find a decent pearl on your dish while eating mussels, however, is nil. There are only a few species of the genus Pinctada that form the precious and highly attractive pearls which are traded as jewellery. Pearls from other species are usually rather dull and irregular, although often quite beautiful as well. Some pictures of pearls from different molluscs can be found at the website of The Field Museum in Chicago (http://www.fmnh.org/pearls/), which is certainly worth a visit for anyone interested in pearls.
A traded pearl of an acceptable size took at least two years to grow. Today, this happens mainly on mussel farms, where pearl-producing mussels are inoculated with foreign pieces of material, so that they form a pearl around it. This practice was already done by the Romans more than thousand years ago. Today's pearl harvest became an actual industry, as 99 percent of all pearls worldwide are coming from breeding stations. Nevertheless, the glamour surrounding pearls is persisting. No matter that they are never as immortal as a diamond (the glossy shining does not remain for much longer than about 150 years), pearls remain appreciated as delicate and, with some exceptions, nowadays also inexpensive jewellery.
It is not advisable to open wild Pinctada in hope to find pearls: It's rather improbable that a parasite makes its way far enough into the mussel (while taking a piece of pearl-forming mantle tissue with it), so that the pearl is really round and not attached to the mussel's shell. Thus, one would have to open large amounts of mussels for the eventual luck to find a decent pearl. Needless to say that these mussel species became rare due to extensive sampling.
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