It is thought that natural pearls form under a set of accidental conditions when a microscopic parasite enters a bivalve mollusk and settles inside the shell.
The mollusk, irritated by the intruder, forms a pearl sac of external mantle tissue cells and secretes the calcium carbonate and conchiolin to cover the irritant.
This secretion process is repeated many times, thus producing a pearl.
Natural pearls come in many shapes, with perfectly round ones being comparatively rare.
Cultured pearls are the response of the shell to a tissue implant. A tiny piece of mantle tissue from another shell is cut into a recipient shell, causing a pearl sac to form into which the tissue precipitates calcium carbonate.
There are a number of methods for producing cultured pearls: using freshwater or seawater shells, transplanting the graft into the mantle or into the gonad, and adding a spherical bead as a nucleus.
Most saltwater cultured pearls are grown with beads, as in the mollusk is cut open and beads are placed/shoved inside the cut. Then they are left alone for a few months, then forced open, cut again and the pearls are removed.
(this is both cool and animal abuse on some levels, I think)
Most beadless cultured pearls are mantle-grown in freshwater shells in China, and are known as freshwater cultured pearls.
Imitation pearls are simply made of mother-of-pearl, coral or conch shell, while others are made from glass and are coated with a solution containing fish scales called essence d'Orient.
Although imitation pearls look the part, they do not have the same weight or smoothness as real pearls, and their luster will also dim greatly. But they come in so many colours.
Most of the pearls I use are imitation pearls, more specifically coated glass.
(Sorry for the nerd explosion)