There are Natural and Cultured Pearls. Technically the pearl should only refer to a natural pearl. Nevertheless, cultured pearls are so common and the natural ones so rare that the word "pearl" now normally refers to a cultured pearl.
Pearls can additionally be classified as saltwater or fresh water.
Although pearls produced by mollusks are a product of nature, it is not normal for an oyster to produce pearls; they result from an intrusion.
Nuclei are the basis of pearl formation. There are various theories, about the origin of pearls, as it seems there is not one basic cause for the formation of pearls, but several. The grain of sand theory can be cited as an extremely rare case indeed. Other intrusions can be a minute snail, worm, fish, crab, clay or mud; as well as self-generated inclusion of the shell-making material of the oyster.
Probably the most important cause of the origin of pearls is that of the parasitic intrusion. It is the result of a morbid condition of the mollusk. The pearl is the tomb of a parasitic worm.
When an irritant object enters its shell, the oyster tries to eject it, but failing this it isolates the intruder either by immuring it against the inner shell wall, thus forming a blister, or by encystation, i.e. putting it in a sac, or cyst. Usually, the intruder is a parasitic worm, which causes a depression on the surface of the mantle, slowly sinking in until it is in a hollow below the surface. Eventually, the hollow is sealed over, the parasite dies, and its skeletal remains receive a coating of conchiolin, which hardens to form a nucleus. From then on, secreted fluids from the epithelial cells of the sac cover the nucleus with overlapping fine films of nacreous aragonite.
If the oyster can move freely, the nucleus receive concentric layers which form that finest of pearls, a spherical cyst pearl. We now have a pearl of radial and concentric structure, chiefly of aragonite crystals radiating outwards from the center. Movement restricted in any particular direction produces a variety of shapes such as oval, drop-shape or button-shape.
The pearl consists of 90 % calcium carbonate with equal proportions of water and organic matter (conchiolin an organic substance, akin to finger nails. Calcium carbonate and a little water (2% to 4%) and chonchiolin make a binding agent.)
Conchiolin is secreted by cells of the mantle, which covers the soft body, viscera, gills and other internal parts of the mollusk. The mantle is responsible for the secretion that forms the outer shell, its mother-of-pearl lining and eventually pearls themselves, which are mainly nacreous. Nacre is the mother-of-pearl layer secreted by certain mollusks and lining most bivalve shells. It consists of crystalline carbonate of lime with organic conchiolin and forms either all or only the outer layers of nacreous pearl.
Natural pearls found in oysters of the Persian Gulf or those that look like pearls found there. e.g. natural pearls found in the Red Sea or the Gulf of Mannar (off the West Coast of Sri Lanka)
Akoya Pearls Sizes ranging from 2 mm to 9 mm, these are saltwater pearls from the Akoya oyster, which is usually cultured. Even though they are often called Japanese pearls, they can also be found in oysters outside Japan. In fact, China has become the major producer of Akoya pearls less then 7mm in size. Korea, Hong Kong and Sri Lanka are already culturing pearls using Akoya oysters.
Biwa Pearls Freshwater pearls cultivated in Lake Biwa, Japans largest lake. Lake Biwa is one of the worlds first freshwater culturing sites and it has been noted for its high quality pearls.
South Sea Pearl Sizes ranging from 8 to 18 mm. As a general term, it can denote any saltwater pearl found in the area extending from Burma and Indonesia down to Australia and across to French Polynesia. It is used more often for large white pearls cultured in the Pinctada Maxima oysters a large oyster found in the South Seas, which is also called the silver-lip or gold-lip oyster.
Black Pearls Dark-colored pearls of natural color (not dyed) from the black-lip oyster. Some people use the term "Black pearl" to refer to any dark colored pearl.
Blue Pearls Dark-colored pearl found in oysters such as the Akoya or silver-lip oysters. The color is due to foreign contaminants in the nacre or between the nacre and shell bead nucleus unlike black pearls whose color is an inherent characteristic of the pearl nacre.
Mother of Pearl The smooth, hard lining on the interior of a mollusk shell, which is used to make decorative objects, buttons and beads.
Half Pearls Whole pearls that have been ground or sawed on one side, usually to remove blemishes. The term "half pearls" sometimes refers to blister pearls (see below).
Blister Pearls Natural or cultured pearls that grow attached to the inner surface of the oyster or mussel shell. When cut from the shell, one side is left flat with no pearly coating.
(Tennessee is a major source of cultured solid blister pearls. Their nacre is very thick due to the fact they are left in the mollusks for three to five years. American pearls are never bleached, dyed or treated.)
Mabe Pearls Assembled cultured blister pearls (pronounced MAH-bay). The blister pearl is cultured by gluing against the inside of the shell a half-bead nucleus (often of plastic or soapstone). After the mollusk has secreted nacre over the bead, the blister pearl is cut from the shell and the bead is removed so that the pearl can be cleaned to prevent deterioration. The remaining hole is filled with a paste or wax (and sometimes also a bead) and then covered with a mother-of-pearl backing. Mabe pearls are not as solid as blister pearls.
Seed Pearls Small, natural pearls that measure about two millimeters or less. They usually weigh less then 0.06 carat.
Dust Pearls Tiny pearls that weigh less than 0.01 carat and which are too small be used for jewelry. These pearls are ground up and used for medicinal purposes in China and South East Asia
Keshi Pearls that grow accidentally in the soft tissue or the adductor muscle of cultured pearl-bearing mollusks. "Keshi" is the term for poppy seed in Japanese. It originated from the minute pearls that were spontaneously formed when a much larger pearl with a bead nucleus was cultured in the Akoya oyster. To the Japanese, these tiny pearls without nuclei resembled poppy seeds, hence the name.
After the larger Akoya cultured pearls are removed from the shell, the keshis are collected. Then they are usually exported to countries where the labor cost of drilling and stringing them is low.
Keshi pearls that are found to be of less luster and are unsuitable for jewelry are ground up and used for medicinal purposes as are the dust pearls.
Today the term "Keshi" also refers to bigger pearls without nuclei that are spontaneously formed in cultivated South Sea oyster and fresh water mussels. These can exceed 10 mm in width and 10 mm in length. Fine keshi pearls are noted for their high luster and irridescence and unique shapes.
Mikimoto Pearls Pearls produced and marketed by the Mikimoto Co. Mikimoto (America) Co., Ltd. Estimates that only the top 3-5 % of all cultured pearls harvested in Japan meet the strict requirements for carrying their trademark clasp. Only those pearls, which have an 18-karat-gold Mikimoto signature clasp, are true Mikimoto pearls.
The chief sources of pearls and mother-of-pearls are the Persian Gulf (Bahrain), New Guinea, Borneo, the Gulf of California, the Gulf of Mannar (Sri Lanka), North Western Sri Lanka, the northern and north western coasts of Australia, Tahiti, the Mergui Archipelago, the Sulu Sea, Venezuela and Mexico.
Since natural pearl production by oyster is entirely accidental and not a normal function, cultured pearls are induced production by potential pearl-bearing oysters, reared in safety and under ideal conditions.
The young oysters, termed "spat", are reared in special areas before being used for pearl cultivation. Between late April and early July and again from mid September to late October,
the spats are collected and are prepared for insertions. The practice is to insert a spherical bead, called the nucleus, together with a cube of mantle tissue from a sacrificed oyster. Generally the bead nucleus is from 5mm to 7mm in diameter but with smaller nuclei several insertions may be made into one oyster. The insertion and placing of nucleus and mantle tissue into the ventral swelling of the gonad (sex organ) is a very skilled and delicate operation.
(The nucleus is mainly a bead from freshwater mussel shell which is acceptable to the oyster, and lacking of luster, does not affect the eventual color of the cultured pearl.)
(Mantle a membranous tissue that secretes nacre and lines the inner shell surface of mollusks.)
Sometimes people are surprised to discover how small oysters are for the pearls they host. Japanese pearl oysters grow to only about 4" in diameter.
Growth times The inserted epithelial tissue will start to multiply, depending upon water temperature, after about 10 days and will form a sac around the bead nucleus and start to secrete and cover the intruding bead. Once the treated oysters have passed a convalescent stage they are transferred to permanent culture rafts from which the cages or net panels are suspended at a depth of 7 to 10 feet. The cages are lifted about three times a year and the oysters cleaned of marine growth. An average cultured pearl takes about 3 ½ years to grow.
Thickness of layers The secretion of nacre from the cells of the pearl sac forms three or four layers per day, but this can vary according to season and temperature. Medium-quality pearls are said to have a thickness of pearl surrounding the bead nucleus of 0.4 to 0.5mm. One thousand layers could take a year, each layer being 0.5 microns in thickness (a micron is one millionth of a meter).
The harvest December and January are the harvesting months because the flow of nacre then is of a finer quality. According to one account, only about one in four oysters prepared with nuclei will produce a cultured pearl after three years, and only one of four of these will be of marketable quality.
Bleaching The cultured pearls are extracted simply by opening the oyster and removing them. They are then washed free from mucus and slime and finally dried. Most Japanese cultured pearls have a hint of greenish hue when first extracted; some are of a darker color but most of this is lost very quickly by a bleaching in weak hydrogen peroxide solution.
Color in cultured pearls is governed by the salinity of the water, richness or excess of conchiolin, temperature and health of the oyster, and the species, i.e. silver-lip, gold-lip and black-lip types of pearl oyster.
Fresh Water Pearls (Lake Biwa, Japan)
Fresh water pearls are found in pearl mussels in rivers, lakes and ponds. These mussels, which are about 9 inches (maximum) in length, live to about 13 to 15 years. At seven years stage they are ready for the culturing of pearls. They reach maturity at 10 to 13 years.
Collection Mussels are collected by trawling the lakebed with rakes which have suitably-sized gaps between the prongs to prevent small mussels being fished.
Insertion of nucleus diced pieces of mantle tissue from a donor mussel are inserted into slits/incisions made in the edge of the mantle of the producing mussel. There have been reports of successfully making 10 incisions on each side of the mussel interior and some of twenty on each side. This, of course, limits the size of cultured pearl produced.
Colors can also be controlled by judicious selection from certain portions of mantle tissue, together with precise positioning within the incised mantle.
Shapes By using hypodermic syringes, airpumps and medicine in their operations, they can control fancy shapes.
The harvest At least 60% are successful and of the successful ones, practically 100 % produce their full complement of 10 to 20 and sometimes up to 40 cultured pearls per mussel. Following the harvesting of the crop, if the mussels are returned to the water, a second crop and sometimes a third crop can be harvested. The second crop is duller and flatter generally than the first, whilst the third crop is rather like flat slices of pearl and, not being very attractive, is sometimes ground up for medicine or used as tomb pearls.
Coating pearls being coated with lacquer. Lacquer will temporarily improve luster but will also eventually wear off over time, leaving buyers feeling deceived if not advised of the coating.
Filling Low-quality baroque pearls are occasionally filled with an epoxy substance if they are partially hollow or have a loose nucleus. This is done to make them more solid and improve durability.
Dyeing White or cream-colored pearls are sometime soaked in pink dye to give them a desirable pink tint. This can often be detected in the drilled holes or in cracks.
Golden pearls may also be dyed.
Irradiation This method works best on freshwater pearls, but off-color Akoya and South Sea pearls may also be darkened in this manner. This involves bombarding pearls with gamma rays, which will blacken the shell bead nucleus of the Akoya and South Sea pearls and makes their nacre appear dark. Sometimes pearls are both dyed and irradiated.
Silver salt treatment This is the most common way of blackening Akoya pearls. The pearls are soaked in a weak solution of silver nitrate and diluted ammonia and then exposed to light or hydrogen sulfide gas. Unfortunately, the silver nitrate tends to weaken the pearls and make them more susceptible to wear.
Dying the nucleus bead Occasionally shell nuclei are dyed before they are inserted in the oyster. Afterwards the dark bead shows through the nacre and makes the pearl nacre look dark.
Imitations come in a variety of types. The main ones are:
Hollow glass beads containing wax These "pearls" are made by mixing pearly substance into a kind of varnish and later coat the inside surface of a hollow glass bead. The bead is later filled with wax to resemble imitation pearls. (This method is mostly found in antique jewelry)
Solid glass bead These may be covered with as many as forty coats of pearl essence and hand polished between each coating. (Example: Majorica imitation pearls.) Imitation glass pearls are also coated with other substances such as synthetic pearl essence, plastic, cellulose and lacquer.
Plastic beads These may have the same type coatings as the glass bead. Plastic imitation pearl necklaces sometimes hang poorly due to their light weight.
Mother-of-pearl beads These are coated with the same substances as plastic or glass imitations. A coating made from powdered mother of pearl and synthetic resin may also be used. They are sometimes called semi-cultured. This is a very misleading term.
Simulated pearls - another term to designate imitation pearls.
Tooth Test Rub the pearls lightly along the biting edge of your upper front tooth. It should feel gritty or sandy, while imitations will feel smooth.
Surface magnification test Using a 10x loupe (magnifier), the surface of an imitation looks coarse while that of the cultured or natural will have a smoother-looking surface.
Flaw Test - If they appear flawless, this is a good sign that they are imitation.
Matching Test Note the shape, luster, and size of the pearls. Imitations often seem perfectly matched, whereas there tend to be variations among cultured or natural pearls.
Heaviness Test Heft the pearls in your hands. If they feel unusually light, theyre most likely made of plastic or filled with wax. Solid glass beads may feel heavier or about the same as cultured and natural pearls.
Drilled Hole Test With a 10x loupe, examine the area around the drilled hole.
Cleaning Pearls (coming soon)
Storing Pearls (coming soon)
Other Handy Tips (coming soon)
Below are some reference books that I have found very
useful. You might like to take a look.
Practical Gemology by Robert Webster