Our ancestors probably first discovered oysters when they were hunting and gathering food along seashores thousands of years ago. The tasty, nutritious molluscs also yielded pearls, and archaeological studies provide evidence that pearls were as highly prized in ancient cultures as precious metals and jewels. They remained highly prized for millenia because, as well as being beautiful, pearls were also rare, with thousands of oysters yielding only one or two of these strange, beautiful creations.
The adult oyster is a sedentary creature, attached to its substrate and unable to move. It depends on filter feeding to capture food and two strong shells to protect it from predators. The shells are secreted by the mantle, a thin layer of tissue that surrounds the body of the oyster like a cloak and which can secrete the two calcium carbonate shells which make oysters and other bivalves like cockles and mussels so distinctive. Clams and mussels can also produce the occasional pearl but it is a much rarer phenomenon in these species. Other molluscs such as snails and abalone also have a mantle but they make only one shell and since they can move sand grains outside the body, they don't make pearls.
It was only in the late 19th century that people worked out why some oysters made pearls, and the vast majority did not. It turned out to be a solution to pollution. When a grain of sand or a piece of food or a bit of shell gets inside the oyster, it irritates the soft mantle and the oyster's other tissues. The mantle responds by building a shell around the irritant, creating a pearl. Pearls are found in a number of oyster species but only a few make the perfectly round pearls that bring the highest prices for their beauty.
The discovery of the sand grain as the stimulus for pearl production led to experiments to create more pearls. Sand grains or pieces of shell were placed inside unsuspecting oysters and the first cultured pearls were created. in the 1930's. Eventually so many cultured pearls were made that the price collapsed and many natural pearl businesses disappeared. Their doom was sealed when cheap plastics replaced mother of pearl for other products but the long term result has been the establishment of pearl farming as a business, the saving of the wild pearl oysters from overharvesting and the creation of a product that is available to many and not just the rich. In addition artificial 'pearls' are also manufactured from glass beads surrounded by wax, plastic and material from fish scales.
So how does the oyster do it? The mantle takes minerals from the oyster's food to make the substance called nacre, which is also called mother of pearl when it is laid down on the inside of the oyster's shell. It is shiny, and usually creamy white or grey, although it can also be shades of pink, red, green, blue or even black, depending on the mix of minerals. Nacre is made up of alternating layers of two substances: aragonite and conchiolin. Aragonite is calcium carbonate (CaCO3) and conchiolin is an organic binding agent made up of amino acids. Conchiolin forms chanbers that hold the pearl nacre. These two substances give the pearl its translucent, shiny appearance.
Not all pearls are perfectly round. Unevenly shaped pearls, called baroques, are also formed. An oyster can secrete a new layer of shell around an irritant at the rate of about 0.1 to 0.2 mm per year so it is a slow process taking up to seven years to form the perfect pearl. Cultured pearls are generally harvested younger than natural pearls so their nacre layers are not as thick. For this reason natural pearls are still worth more than the cultured variety.
Want more information? Check out these websites: http://animals.howstuffworks.com/marine-life/question630.htm and http://www.allaboutgemstones.com/pearl_composition.html