Pearl cultured

The growth mechanism, form, and texture of aragonite crystals in the shells of bivalves and snails or in pearls have been extensively investigated over many years in order to control the quality of cultured pearls [6],... 

Pearls are little spheres of calcium carbonate (CaC03) that form in mol-lusks (invertebrate shellfish) such as oysters, usually because of some sort of irritation. They are normally white or off-white in color, but they can have bluish or pink tints, and sometimes they are dark gray. Although many pearls form naturally, pearl production has been gready increased by the cultured pearl industry, which raises beds of oysters into which irritants are routinely introduced. The irritants are usually bits of mother-of-pearl, the lining that forms inside oyster shells. 

Natural or oriental pearls are produced naturally by bivalves, such as oysters and clams, and by a few gastropods, such as abalone. Cultured pearls are induced by the introduction of foreign objects into the mantle cavity. Modem cultured pearls are usually seeded with a sphere made from the shell of Mississippi River mussels (family Unionidae). The culturing process involves prying open the shell and inserting both the seed or nucleus that will become the core of the new pearl, and a piece of donated mande tissue from another oyster. 

The longer a cultured pearl stays in the oyster, the more layers of nacre there will be. If a pearl is harvested too soon after the nucleus is deposited, the layers of lustrous nacre may be so thin that they wear away when the pearl is used in jewelry. 

Cultured and natural pearls can usually be distinguished from each other with the use of a medical x-ray. Cultured pearls will show a core bead of flat layered shell with thin layers of nacre around it, while natural pearls will be concentric throughout. 

Cultured pearl one that is artificially induced by the placement of a foreign body within a mollusk, usually an oyster. 

All pearls can occur naturally, but today the majority of them are cultivated and are known as cultured pearls . Although they grow naturally inside a mollusc, their growth is instigated and controlled by human intervention. 

Pearls with a nacreous sut ce can be divided into three groups natural pearls, nucleated cultured pearls, and non-nudeated cultured pearls. (Pearls without nacre will be dealt with separately at the end of the Spedes secdon.) A further group - blister pearls - are hemispherical pearls that grow attached to the oyster s shell. These can also be cultured. 

Nucleated cultured pearls, as the name implies, consist of solid nuclei (usually mother-of-pearl shell beads) with an overlying skin of nacre (Fig. 9.1). As with shell nacre, the organic matrix secreted by the mollusc forms a framework over the bead, in which the aragonite crystallises. This organic material seems to be concentrated at certain growth stages and also to be influenced by seasonal change. This fact aids the identification of the pearls. For example, it is the cause of the demarcation seen between the nacre and the bead in a nucleated cultured pearl. 

Non-nucleated cultured pearls have a similar structure to that of natural pearls, but at their core there is an irregular cavity, which is a result of the initial stages of their growth which was induced by humans. 

Figure 9.4 South Sea nucleated cultured pearl (magnified).

Tlie black-lipped pearl oyster, Pimtada margarm a, is a little smaller, and measures fiom 15 to 25 centimetres across. It, too, has silvery nacre inside the shell, becoming silvery gr at the edges, which are surrounded on the lip of the shell by a black border that resembles enamel. The oyster lives in the Indian Ocean and the western and central Pacific, eq>ecially around French Polynesia. It is used to produce the fiunous Tahitian black pearis, which are nucleated cultured pearls. The nacre secreted by these pearl oysters is naturally coloured and varies om li t gr to black, with iridescent hints of green, purple, pink and blue. The pearls rai in size from 10 to 18 millimetres in diameter. 

The Akoya pearl oyster, Pmctada Jucata moitenm, is again smaller, measuring 8 to 10 centimetres across. It has silvery nacre inside the shell. Akoyas are used in Japan to produce their world- mous nucleated cultured pearls, and are now also being used in China and other parts of the Far East. The cultured pearls produced by these oysters vary in size fiom 2 to 10 millimetres in diameter, and are white to cream in colour. 

The Biwa pearl mussel, Hyriopsis schlegelii, is probably the most famous of the freshwater mussels. It was native to Lake Biwa in Japan, though the stocks are now very depleted. It was used in the production of Biwa pearls, which were the first freshwater, non-nucleated cultured pearls to come on the market. They were of an irregular oval shape. 

The cockscomb pearl mussel, Cristaria plicata, lives in Japan and China. It produced the original rice-krispie pearls. They were small, irregular in shape, and had a wrinkled appearance. They were naturally white, but were frequently dyed. The mussels have thin shells and, when pearl cultivation was first attempted in China, were proved to be incapable of producing freshwater cultured pearls of high enough quality. 

The triangleshell pearl mussel, Hyriopsis cumingii, is the oyster now used in China to produce their freshwater cultured pearls, which are of a vastly superior quality to their original rice-krispies . The triangleshell is also farmed in Japan. It has a much thicker shell than the cockscomb, and it can produce nearly perfect round pearls with a good lustre. The naturally coloured nacre occurs in whites, orange-browns and blue-greys. White is the most popular colour (Fig. 9.5). 

The term liaroque pearl is used to describe an irregularly shaped pearl. It was first applied to irregular natural pearls, but can also be used to describe the irregular shapes seen in cultured pearls. Baroques can be very beautifiil and are highly prized (Fig. 9.6). 

There are many natural and cultured pearls of interesting shapes that come onto the market. The freshwater mussels of the eastern United States produce natural pearls that are usually silvery ute> often have a slighdy silky sheen rather than a pearly lustre and can be an amazing variety of shapes (Fig. 9.7). 

Cirdte pearls are nucleated cultured pearls from marine pearl oysters. The round to oval pearls display indented rings around the middle of the pearl. It is not known eatacdy why this patterning occurs. Two possible theories are that the mantle tissue was twisted when inserted into the oyster, or that the pearl turned firequendy while it was growit. ... 

The thickness of a pearl s nacre depends not only on the length of time the oyster is left in the water, but also upon the health of the host oyster, the surrounding water conditions, and the quality of the bead nucleus used. The season at which the cultured pearls are harvested also has an influence on the lustre of the nacre. 

Many years after the original pearl cultivation was started, it was realised that it was not necessary to insert a hard nucleus into an oyster to induce it to produce a pearl, and that it was sufficient simply to insert a piece of mantle. This is the method favoured by the Chinese, and gives non-nucleated cultured pearls . The pearls are grown in freshwater mussels, suspended from floats in lakes and rivers. Up to 30 mantle grafts are inserted into a single host mussel s mantle tissue, just inside the shell. 

The original attempts at Chinese pearl cultivation produced pearls which were relatively small and wrinkled. They were generally referred to as rice krispies after a well-known breakfast cereal of a similar shape. Nowadays much better quality non-nucleated cultured pearls are produced in China (Fig. 9.5). 

With all types of cultured pearl, the host oysters and mussels must be tended during the cultivating process. They are regularly lifted from the water and cleaned to remove algae and barnacles that may have accumulated. When the appropriate time has passed for the pearls to have reached their optimum quality, they are harvested. The oysters or mussels are opened and the pearls are extracted. 

F pire 9.13 Nucleated cultured pearl destroyed by bleaching. 

Figure 9.15 Nucleated cultured pearl vrith broken nacre.

Note Non-nucleated cultured pearls of poor quality are nowadays plentiful and cheap. It is likely that they will replace imitation pearls on the market as there will be no economic advantages in producing pearl simulants. 

Figure 9.17 Cultured pearl, showing bead nucleus.

A collection of pearls that are perfectly matched in size would similarly indicate cultured pearls. 

Fignre 9.19 Dyed nucleated cultured pearl, sfanwing accumulatioa of dye. 

A collecdoQ of drcl6e pearls would suggest nucleated cultured pearls. Girdles are rare as natural pearls. 

A nucleus of a different colour to the nacre is an obvious sign of a nucleated cultured pearl. A dark bead beneath white nacre will give the impression of a grey pearl. It is probable that the pearl has been irradiated as this process darkens the freshwater mother-of-pearl bead rather than the nacre produced by a marine oyster. It is also possible that the bead has been dyed. 

A dyed pearl, whether natural or cultured, may show concentrations of colour in surface imperfections, or between nacre layers when viewed down the drill hole. This may be especially evident in nucleated cultured pearls where the dye can accumulate between the mother-of-pearl bead and the overlying nacre (Fig. 9.19). 

As a result of wear and tear, the nacre on a nucleated cultured pearl can occasionally become chipped. This is most likely to be apparent around the drill hole (Fig. 9.20). 

For further tests it is necessary to seek the help of a gem-testing laboratory, where they will use equipment not available to the general public. An earlier, popular test was called a Lauigram, which was an X-ray diffraction test taken of each individual pearl, in which concentric layers of crystals gave one result, while a pearl with a mother-of-pearl bead nucleus gave another result, due to the bead s parallel layers of crystals. The advent of non-nucleated cultured pearls, which have the same crystal formation as natural pearls, has resulted in this test being unreliable for the detection of natural pearls. 

Non-nucleated cultured pearls may show the same rings as a natural pearl, but with evidence of an irregular cavity in the centre, caused by the mantle tissue graft. 

Nucleated cultured pearls should reveal a complete two-dimensional circle near the surface of the pearl, caused by the organic matrix between the mother-of-pearl bead and the nacre. 

The famous Biwa cultured pearls, named after the lake where they were farmed in Japan, were almost completely exterminated in the 1970s and 1980s. This was caused by a combination of factors, which included pollution from agricultural waste, overfishing, and a freshwater form of red tide. It is taking years to restock the lake with the mussels needed to produce these pearls. 

In the mid-1990s the Akoyas in Japan were struck by a virus that reduced cultured pearl production by 75 per cent, seriously damaging the cultured pearl industry of that country. 

Although at first very unpopular and shunned as artificial, cultured pearls soon became popular and took over from natural pearls. Without careful examination and testing, it was often not possible to see the difference, so people were no longer prepared to pay for natural pearls when the cultured ones looked as good. 

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