Both of my books on gemstones call pearls a gem; in fact, one of the earliest-known gems, since they do not require cutting or polishing like gemstones that are actual rocks. Pearls are primarily calcium carbonate (the same stuff as in some antacids) and can be made by oysters, mussels, clams, abalones and other mollusks both saltwater and freshwater. (Not all mollusks' pearls are as iridescent as gem pearls, though.)

Pearls have been harvested as far back as anyone knows, but the process of making cultured pearls was invented in 1893 by Kokichi Mikimoto of Japan. Cultured pearls are produced by the animals in the same way as natural pearls, but they are started off by a person inserting a mother-of-pearl bead into the oyster. This process met with resistance from gem buyers at first, but now the majority of pearls available for sale are cultured. (Natural pearls are much more expensive.)

Pearls are valued on their luster, their size, their shape, their surface, and their color. Pearl sizes are measured in millimeters across, and their weight is expressed in grains. Pearls come in all sorts of colors; the most valuable are white, cream, rose, and black (and sometimes pearls are dyed to try and achieve these colors).

Pearl jewelry is quite delicate (the layer that makes the iridescence and color is often less than a millimeter thick). Don't allow them to come in contact with chemicals such as hairspray and don't store them where other jewelry can scratch their surface. It's also a good idea to have a pearl necklace restrung every few years.