Tahitian cultured pearls
Definition
The Tahitian cultured pearl is a naturally colored cultured pearl resulting from grafting and breeding in a natural environment, in French Polynesia, of the pearl Pinctada margaritifera var. cumingii. It results from the secretion of nacre by a graft (piece of epithelium of the mantle collected from the donor oyster from French Polynesia) around a bead inserted in the gonad of this pearl oyster.
The pearl-producing mollusk
Tahitian cultured pearls grow inside the black-lipped oyster Pinctada margaritifera. Pinctada margaritifera can reach a weight of 5 kilograms and a size of up to 30cm, which is nearly twice the size of a Japanese Akoya oyster. The life expectancy is up to 30 years. The shells are larger in the southern part of the French Polynesia, and can produce larger pearls. The mother-of-pearl has a light grey to silvery grey color. They are found in French Polynesia, Cook Islands, Panama, the Gulf of Mexico, etc.
Breeding takes place from October to February. One matrix aged five to six can produce a total of 10 million eggs or respectively about 100 million sperm. The pearl cultivation process begins with the collection of these small pearl oysters called spat. When the spat are large enough to be identified, they will be placed in spat bags allowing their continued growth. After reaching a size of 5 to 10 cm, they are sold to pearl farms.
The oysters are kept until they are about the size of an adult’s hand or about 13cm. At that point they are mature enough to endure the grafting process.
The seeding process
The oysters are supposed to be two to three years old, 10 to 15cm and in good health for pearl seeding operation.
Firstly the oysters are removed from the water, cleaned, pried open gently and pegged with a small wooden peg. The technician holds the pegged oyster in a stand and makes a long narrow incision through the gonad. A 1 to 2cm piece of mantle tissue form a donor oyster is inserted. The nucleus should be a perfectly round piece of freshwater mussel shell with a highly finished surface.
Once the operation is completed, the oysters are placed inside a ‘catch bag’ and are taken for a brief period to an underwater platform near the shore. About 30% of the operated oysters die and about 40% eject the nuclei.
Oysters which eject nuclei are often operated again after several months. And then a new, larger nucleus is inserted into the pearl sack which can be used for production of larger pearls.
The harvest
A pearl sack forms several weeks after the grafting process, and nacre secretion starts after around 50 days. Between three and eight layers of platelets are secreted in one day, and the thickness of one layer is around 0.5 to 1 micron. The normal growth time is between 18 and 24 months.
After two years, an average nacre thickness of 2mm to 2.5mm can be observed. Growth is linear and constant until the thirteenth to fourteenth month, after which it slows down.
Harvests take place at the same time as seeding and the tropical climate cause overlapping cycles of grafting and harvesting, which enable three to four harvests per year.
The first seeding yields about 50 to 60% of pearls, which means half of the oysters without objecting nucleus that produce pearls of acceptable quality immediately. Only 2 to 3 per cent of all pearls are very good quality. A second operation maybe followed with the pearl sack opened and the pearl was taken immediately. A new and larger nucleus is inserted into the already existing pearl sack. The oyster is returned to the water and cultured for about 18 to 24 months. Nuclei for the second operation are 10mm to 11mm in size.
Grading
There is no uniform grading system for Tahitian cultured pearls, too. People use A-D pearl grading system in pearl industry. This grading system begins with A being the highest grade. And it is used in French Polynesia to grade Tahitian pearls and South Sea pearls only. So it is called ‘Tahitian system’ as well.
A: Perfect surface or slight growth characteristics, luster is very high;
B: the total growth characteristics are no more than 30% of the surface, the luster is high;
C: The total growth characteristics are no more than 60% of the surface, the luster is high;
D: The total growth characteristics are more than 60% of the surface, the luster is low to not present at all.
Color and overtone
The conchiolin substance is host to the coloring agents. Normally, conchiolin is colorless; the dark brown to black color comes from deposits of red and brown organic pigment, carotinoids and porphyrines. The final color of a pearl depends on the type and thickness of the prismatic and nacreous layers.
There is no general system for color and overtone grading. But they are important quality factor. The general rule for Tahitian pearl is the pearls with overtone are valued higher than pearls without overtone, and several overtones combination will add up to a higher price than only one overtone.
Black pearls with a green overtone or the “peacock” pearls are valued highest. A purplish-pink overtone is valued next highest. The combination of pink and blue overtones even though has got the same name as the purplish-pink overtone which called “aubergine”, is not valued at the same price.
