Pearls are the only gemstones made by living creatures. Mollusks produce pearls by depositing layers of calcium carbonate around microscopic irritants—usually a grain of sand, as it’s commonly believed—that get lodged in their shells.
While any shelled mollusk can technically make a pearl, only two groups of bivalve mollusks (or clams) use mother-of-pearl to create the iridescent “nacreous” pearls that are valued in jewelry. These rare gemstones don’t require any polishing to reveal their natural luster.
Appropriately, the name “pearl” comes from the Old French perle, from the Latin perna meaning “leg,” referencing the leg-of-mutton shape of an open mollusk shell. Because perfectly round, smooth, natural pearls are so uncommon, the word “pearl” can refer to anything rare and valuable.
The rarest and most expensive pearls are natural pearls made in the wild without human interference. The majority of pearls sold today are cultured or farmed by implanting a grafted piece of shell (and sometimes a round bead) into pearl oysters or freshwater pearl mussels.
Pearls are very soft, ranging between 2.5 and 4.5 on the Mohs scale. They are sensitive to extreme heat and acidity. In fact, calcium carbonate is so susceptible to acid that authentic pearls will dissolve in vinegar.
The finest pearls have a naturally reflective luster, making them appear creamy white with an iridescent sheen that casts many colorful hues. Cultured freshwater pearls can also be dyed yellow, green, blue, brown, pink, purple or black.
Black pearls—which are mostly cultured because they are so rare in nature—aren’t actually black but rather green, purple, blue or silver.
Pearls used to be found in many parts of the world, but natural pearling is now confined to the Persian Gulf waters near Bahrain. Australia owns one of the world’s last remaining pearl diving fleets and still harvests natural pearls from the Indian Ocean.
Today, most freshwater cultured pearls come from China. South Sea pearls are cultured along the northwestern coastline of Australia, the Philippines, and Indonesia.
If you want to ensure that you get a genuine, high-quality pearl, find an American Gem Society jeweler near you.