The oyster that grows the pearls, Pinctada margaritifera, is extremely abundant in Polynesian waters. The shells of the oyster have been used since ancient times to make jewelry, fish hooks, and lures. In addition, the European button industry used a large amount of the shells prior to the invention of cheaper means to make buttons. Black pearl cultivation now takes place on about 30 islands in the South Pacific, and has largely replace copra (coconut meat) and fishing as a source of revenue.
The pearls produced by the Pinctada margaritifera vary in color from pearly white to nearly black, and include purple, gray, champagne, and greenish colors. There are no pearls that are completely black in color like an onyx.
Making pearls is an arduous process, and not just for the oysters! Either divers collect three to five year old oysters to be used for the pearl cultivation, or they are raised by the pearl farmer. The farm we visited "grew" their own oysters. At certain times of the year, the oysters release sexual substances that are fertilized in the water. These young oysters, known as seed oysters, swim around for a few weeks before attaching themselves to coral. The pearl farmer catches these seed oysters and attaches them to underwater rearing lines like those at the right. Unfortunately, like most farm crops, you can't then ust sit back and watch the oysters grow. The farm family must remove the oysters from the water about every three months and wash them with a spray hose to remove any algae that has grown on the shells and prevent the algae from killing the oysters At this farm, they were cleaning oysters continually, since it took about three months to go through the entire crop!
When the oysters are three to five years old, they are ready for cultivation. First, the farmer sacrifices a donor oyster and divides its mantle into about 50 small particles called grafts. The recipient oyster is pulled from the water, fixed to a support and held open with forceps. (see the oyster in the support in the picture at the left) The grafter, usually a Japanese expert, uses a scalpel to cut the back of the reproductive organ (gonad) of the oyster and insert the graft from the donor oyster. The grafter then inserts a 6 mm perfectly spherical bead, called a nucleus, into the gonad so that it is in contact with the graft.
The grafted oysters are placed in small bags linked together to form "keep nets" (to protect them from predators) and lowered back into the lagoon, where they are regularly cleaned and inspected. (The small bags will also enable the farmer to see and retain any nuclei that have been expelled by the oysters.) If the procedure is successful, the graft cells develop around the nucleus to form the pearl sack. I'm standing next to a pearl sack in the picture at the right. The graft acts as an irritant, and the sack secretes the nacre (mother-of-pearl) material which forms the layers of the pearl. It normally takes from about 18 months for a 1.5 mm layer of mother-of-pearl to surround the nucleus. Sometimes the grafter implants a second nucleus to grow a larger pearl of about 15 to 20 mm. The process for the larger pearls takes about three years.
Of every 100 oysters implanted, 30 do not survive the procedure, and 30 reject the nucleus. When harvest time comes, only 5 of the remaining 40 oysters will have produced perfect pearls. For those oysters that receive the second graft, the success percentage increases to about 10 percent. No wonder the price for an individual pearl can range from $50 USD to $2500 USD, depending on the size, luster, sheen, color, and lack of defects!
Two other products result from pearl farming. The first is called keshi, and is pure mother-of-pearl without a nucleus. Oftentimes the oyster rejects the nucleus into its protective bag but continues to secrete nacre. The resulting pearls vary from 2 to 8 mm in diameter and are often baroque in shape. While we were at the pearl farm, a randomly-selected oyster was sacrificed in order to demonstrate the techniques. Our oyster had a keshi inside. See the photo at the left.
The second product from pearl farming is called mabe. The grafter inserts a plastic mould on the inner surface of the oyster's shell. The mould will gradually be covered with layers of mother-of-pearl. After a few months, a diamond disk is used to cut off the mother-of-pearl. The mabe is hollow and is filled with epoxy resin and used to make jewelry. The exhibit we saw at the pearl farm had many shells that looked like they had growths lining the shells. These were mabe. If you click on the picture to the right and above, you can see some of the shells have mabe growing in them.
Glossary of terms
Baroque: An industry expression used to describe any pearl that is not symmetrical. Typically baroque pearls, whether cultured or natural, will be free form and asymmetrical.
Button: A shape of a pearl where one side of the pearl is slightly flat.
Circles: Concentric rings that form on
the surface of a pearl that are concave in appearance. If circles are apparent
on more than 1/3 of a pearl's surface then the term "circle pearl" is
applied to describe the shape of a pearl. If less than 1/3 of a pearl's surface
is covered by circles, then the circle is considered a blemish and not a shape
Clean: A term used to describe the absence or relative absence of any blemishes on the surface of a pearl.
Color: A quality/value evaluation category used to describe the color of a pearl. Although color is not particularly an indicator of quality, generally creamy/yellow hues are less valuable than other pearl colors
Conch pearl: A pearl produced by a
conch, a saltwater mollusk found in tropical waters. Usually conch pearls
exhibit orange and pink colors and tend to look similar to pink coral.
Cultured pearl: Any pearl which is grown by a mollusk that contains either a hard bead nucleus or soft tissue nucleus at its center, which has been surgically implanted in a mollusk by human means.
Cultivated pearl: Any pearl which is grown by a mollusk that contains either a hard bead nucleus or soft tissue nucleus at its center, which has been surgically implanted in a mollusk by human means.
Grafting: The process of nucleating a mollusk to produce a pearl. Also termed as nucleation or implantation, grafting requires the human insertion of either a hard bead nucleus or soft mantle tissue into either the body of a mollusk or the mantle tissue of a mollusk. The nucleus or mantle tissue serves as a "seed" or "irritant" to produce a cultured pearl.
Gold-lip oyster: A large species of oyster, from the Pinctada maxima family, that can produce cream to gold-colored South Sea pearls. Gold-lip oysters can be found in the waters off Australia, Indonesia, the Philippines, and Japan.
Imitation pearl: A simulated pearl manufactured entirely by man or machine.
Keshii pearl: A keshii pearl, sometimes referred to as a seed pearl, is a non-nucleated pearl. Keshii pearls form by accident in mollusks (usually saltwater mollusks) as a by-product of the culturing process and thus cannot be considered natural pearls. Keshii pearls can be as small as 1 mm and as large as 10 mm in size.
Lustre: A quality evaluation category used to describe the combination of surface shine (reflectivity) and inner light refraction (depth). Lustre is perhaps the most important of all quality factors and is expressed in terms of high, medium and low lustre. The lustre of a high quality pearl should be bright and capable of sharply reflecting objects near its surface. A dull or chalky lustre indicates poor quality.
Mabe pearl: A half-spherical cultured pearl grown on the inside shell of a mollusk, as opposed to inside a mollusk's body. Mabe pearls are grown by glueing a plastic hemisphere onto the inside of a mollusk's shell. Once the hemispherical nucleus is covered with a sufficient amount of nacre, the pearl is cut away from the inner shell, the bead taken out, and the cavity filled with a substance such as epoxy resin and backed by a mother-of-pearl plate. Mabe pearls are sometimes referred to as blister pearls.
Mantle tissue: The thin tissue membrane that attaches a mollusk to its inner shell. Small pieces of mantle tissue are inserted next to hard bead nuclei to help cultured pearls grow in saltwater oysters, while pieces of mantle tissue are used exclusively (without a hard bead nucleus) in many freshwater mussels to grow cultured pearls.
Matching: The process of matching pearls in terms of lustre, surface, shape, color, and size to assemble a necklace or other piece of pearl jewelry.
Millimeter: A metric measurement of length used to determine a pearl's size. Often expressed as "mm," whereby one "mm" equals 1/25 of an inch.
Nacre: A calcium carbonate-based crystalline substance secreted by mollusks to form mother-of-pearl, pearls, and cultured pearls. Nacre secretion by a mollusk is usually a defense mechanism triggered by the intrusion of a foreign object into the body of an oyster.
Nucleus: Typically a small, round piece of polished shell from an American freshwater mussel used as an irritant or core in all saltwater cultured pearl production. In freshwater cultured pearl production, a nucleus is usually a small piece of soft mantle tissue from another freshwater mussel.
Shape: A quality evaluation category used to describe the shape of a pearl. The most valuable pearls are round. However, other shapes of pearls include off-round, drop, oval, button (one flat side), circled, semi-baroque and baroque (asymmetrical/freeform). Freshwater cultured pearls are grown in all of the aforementioned shapes as well as stick, angel-wing, cross and coin (flat on two sides) shapes.
Size: A quality/price evaluation category used to describe the size of a cultured pearl. Size descriptions are expressed in millimeter and measured by the diameter of a pearl.
Tahitian cultured pearls: Cultured pearls produced by the black-lip oyster (Pinctada Margaritifera) found in the atolls and lagoons of French Polynesia. Tahitian cultured pearls are natural in color and are produced in hues of silver, gray, green, orange, gold, blue, purple and black.