Pearls are organic gemstones that form within the soft tissues of certain mollusks. There are two distinct methods of obtaining pearls: natural harvesting, an ancient and rare practice, and cultured pearl farming, which accounts for the vast majority of pearls available today. Each method involves unique processes, from how the pearl originates within the mollusk to its eventual extraction.
Harvesting Natural Pearls
Historically, natural pearls were retrieved through perilous diving expeditions. Divers, often free-diving, would descend to significant depths, sometimes over 100 feet, holding their breath for extended periods to collect pearl-bearing mollusks from the seabed. This method was dangerous, posing risks like drowning, shark attacks, and decompression sickness. The discovery of natural pearls was largely a matter of luck and effort, as only a small fraction of wild mollusks contain them. Due to their scarcity and the dangers involved, commercial natural pearl harvesting is now rare, largely replaced by cultured pearl production.
The Cultivation and Harvesting of Saltwater Pearls
Saltwater pearl cultivation involves a surgical procedure known as grafting or nucleation. This process begins by inserting a small, polished bead, typically made from freshwater mussel shell, into the gonad of a marine oyster. Alongside this bead, a tiny piece of mantle tissue from a donor mollusk is also implanted. The mantle tissue is crucial because its epithelial cells multiply to form a “pearl sac” around the inserted bead, which then secretes nacre, the iridescent substance that forms the pearl.
After the grafting operation, the oysters are returned to marine farms, often suspended in nets from rafts in monitored waters. They remain there for a growth period that can range from several months to several years, depending on the pearl type. Akoya pearls, produced by the Pinctada fucata oyster, are typically harvested after 10 to 14 months. South Sea pearls, grown in Pinctada maxima oysters, can take up to two years to develop. Tahitian pearls, cultivated in the Pinctada margaritifera, have a growth period of around 18 months. When ready, the mollusks are opened, and the pearls are extracted, sometimes allowing the mollusk to be re-nucleated for another pearl.
The Cultivation and Harvesting of Freshwater Pearls
Freshwater pearl cultivation differs from saltwater methods, primarily in the mollusk type and nucleation process. Freshwater pearls are predominantly grown in freshwater mussels, such as Hyriopsis cumingii. Instead of a hard bead, small pieces of mantle tissue from a donor mussel are inserted into the host mussel’s mantle. This tissue stimulates the formation of a pearl sac that produces nacre, resulting in a pearl composed entirely of nacre without a central bead.
Freshwater mussels can yield multiple pearls per harvest. Farmers often implant numerous tissue pieces, potentially resulting in up to 50 pearls from a single mussel, although typically around 12-16 pieces are used for better quality. The growth period for freshwater pearls usually ranges from 3 to 5 years. This higher yield and the tissue-only nucleation method contribute to the greater volume and affordability of freshwater pearls compared to their saltwater counterparts.
Initial Steps After Harvest
Once pearls are removed from the mollusks, they undergo processing for further evaluation. The initial step involves cleaning to remove any residual tissue, mucus, or grime that may have adhered during formation. This is often achieved by tumbling the pearls in rotating barrels with salt or washing them in saltwater or a mild cleaning solution.
Following cleaning, many pearls are polished to enhance their luster and smooth out minor imperfections. This can involve tumbling them with natural materials like bamboo slivers, eucalyptus leaves, or ground walnut shells, sometimes with an oily medium such as beeswax. Initial sorting then occurs, separating pearls by basic attributes like size and shape using sieves. These first steps ready the pearls for more detailed grading and eventual use in jewelry.