Pearls are the oldest gemstone, and for hundreds of years were considered the most valuable. The earliest pearls were formed naturally when a foreign object - a piece of grit, say - got inside the shell of an oyster or a freshwater mussel. To protect itself from the irritant, the oyster or mussel would secrete layer upon layer of a substance called nacre. Over time, the secretions would completely encircle the irritant and form a deep iridescent covering, resulting in a pearl. Prior to the 20th century, pearls were among the rarest and most precious of gemstones. A necklace of matched round pearls was a piece of indescribable value. Today, however, jewelry-grade pearls very seldom occur without human intervention.
At the turn of the 20th century, two Japanese inventors - a biologist named Tokichi Nishikawa and a carpenter named Tatsuhei Mise - independently invented ways to cultivate pearls by implanting foreign objects into an oyster's body. Another inventor, Koichi Mikimoto, discovered a similar but improved technique and bought the rights to Nishikawa's and Mise's patents, thus making his own name the most famous in the pearl industry. Today, Mikimoto continues to be a well known brand name (not a type of pearl, as many people believe). Interesting, at the core of every Japanese saltwater cultured pearl is a small, white bead nucleus made from the shell of the American Pig Toe clam.
Over time, other experts have discovered ways to enhance cultured pearls. They're often mildly bleached, for even coloration, or dyed to produce overtones or stronger colors. Natural pearls are rarely seen on the market today. When they're available, they're usually vintage pieces, and are sold by carat weight. More common cultured pearls are normally sold according to their size, color and quality.
Graduate Gemologist, GIA
Certified Appraiser