Most gems result from the cutting and polishing of precious or semiprecious stones. Only one type of gem is created naturally by a living animal: the pearl.

Pearls form as solid masses when matter accumulates inside the shells of certain types of bivalve (two-shelled) mollusks. Saltwater pearls are formed from pearl oysters. Pearls of different oyster species vary in color and size. Examples of pearl oysters include the Akoya pearl oyster (Pinctada fuacata), the white-lipped oyster (Pinctada maxima), and the black-lipped oyster (Pinctada margaritifera). The iridescent inner portion of the shell is composed of a substance called nacre, which is commonly known as mother-of-pearl.

Freshwater pearls are produced by several varieties of freshwater pearl mussels. One of the most commercially important is the Triangle Sail Mussel (Hyriopsis cumingii). Another variety with historical significance, the Freshwater Pearl Mussel (Margaritifera margaritifera), was especially noted for the quality of its pearls. It is currently listed as an endangered species.

Natural pearls form randomly and rarely. Today, most commercially available pearls are cultured. The culturing process instigates the formation of pearls through controlled conditions whereby a piece of shell is used to create a nucleus around which a pearl will form. Pearl farmers insert the nucleus into an appropriate host bivalve, typically a mussel or pearl oyster, to instigate the formation of a pearl. Cultured pearls should not be confused with imitation pearls, which are created from man-made substances, such as glass or plastic, and sometimes coated with mother-of-pearl.

Pearl-producing oysters are not related to the type of oyster commonly consumed as food. Edible oysters belong to the Ostreidae family of bivalves, within which there are five major genera: Crassostrea, Hyotissa, Lopha, Ostrea, and Saccostrea. The common European Oyster is Ostrea edulis, and the oysters of Chesapeake Bay are Crassostrea virginica. Although edible oysters can occasionally make small, irregular pearls, they are not valued as gems. Ostreidae shells are comprised of calcite and lack the necessary compounds to create the luster associated with gem-quality pearls.

Pearls have been worn as jewelry since antiquity. One Hindi legend tells the story of Krishna giving his daughter a pearl. A Chinese story dating from the 23rd century BC mentions strings of pearls. During the first century BC, pearls were worn by Roman nobles. The Bible mentions pearls in both the Old and New Testaments.

Pearl is one of three gems identified as a birthstone for the month of June. The other two are moonstone and alexandrite. Moonstone is part of the feldspar family. It comes in a variety of colors and displays an optical phenomenon known as adularescence, which creates an illusion of movement. Alexandrite, a newly discovered gemstone, was first discovered in the Ural Mountains in the 1830s and named for Tsar Alexander (1818-1881). Alexandrite changes color based on the source of lighting. In daylight, it appears green or blue-green, but under incandescent light it appears red or purple.

This article, along with images and resources, originally appeared in the June 2016 issue of Observations: A Pier Press® Newsletter. To view the complete newsletter, visit our online archives. Also, please consider becoming a subscriber so that future editions can be delivered directly to your inbox. Subscriptions are free!