Some types of oysters are highly prized as food, both raw and cooked. Other types, such as pearl oysters, are not commonly eaten.

True oysters, belonging to the family Ostreidae, are incapable of making gem-quality pearls, although the opposite idea is a commonly-encountered misapprehension, often seen in illustrations or photographs where an edible oyster shell is mistakenly paired with a gem-quality pearl.

True oysters

True oysters are members of the family Ostreidae. This family includes the edible oysters, which mainly belong to the genera Ostrea, Crassostrea, Ostreola or Saccostrea. Examples are the Belon oyster, eastern oyster, olympia oyster, Pacific oyster, Sydney rock oyster and the Wellfleet oyster.

Pearl oysters

Pearl oysters however are not closely related to true oysters. They are in a totally different family, the Pteriidae (Feathered Oysters). Both cultured pearls and natural pearls can be obtained from these oysters, though other molluscs, such as the freshwater mussels, also yield pearls of commercial value.

The largest pearl-bearing oyster type is the saltwater Pinctada maxima, which is roughly the size of a dinner plate. Not all individual oysters produce pearls naturally. In fact, in a haul of three tons of oysters, only around three or four oysters produce perfect pearls.Fact: date=June 2007

In nature, pearl oysters produce natural pearls by covering a minute invading parasite with nacre. Over the years, the irritating object is covered with enough layers of nacre to form what is known as a pearl. There are many different types and colours and shapes of pearl; these qualities depend on the natural pigment tone of the nacre, and the shape of the original irritant which was being covered over.

Pearls can also be cultivated by pearl farmers placing a nucleus, usually a piece of polished mussel shell, inside the oyster. In three to six years, the oyster will produce a perfect pearl. These pearls are not as valuable as natural pearls, but look exactly the same. In fact since the beginning of the 20th century, when several researchers discovered how to produce artificial pearls, the cultured pearl market has far outgrown the natural pearl market. Natural pearls have become increasingly scarce and a necklace with only natural pearls can easily cost several hundred thousand (US) dollars.

Other types of oysters

A number of bivalve mollusks other than edible oysters and pearl oysters also have common names that include the word "oyster", usually because they either taste or look like oysters, or because they yield noticeable pearls. Examples include:

• the family Spondylidae, the thorny oysters;
• the Pilgrim oyster, a kind of scallop.
• the Saddle oyster (Anomia ephippium)