The expression “South Sea pearl” is a commercial term used by pearl farmers and dealers. This name, so evocative of tropical seas, refers to the large pearls produced by two species of pearl oyster, or pteridean, of considerable size, which live in these warm seas. The white-lip pearl oyster, Pinctada maxima, includes two varieties, the gold-lip, which gives golden coloured pearls, and the silver-lip which gives white pearls. The black Pinctada margaritifera, whose deep-sided shell
measures about 25 cm, produces the black pearl.

 

Exceptionally large baroque pearl harvested from pinctada maxima

 

A large black produced by a Pinctada Margaritifera

To succeed in growing cultured pearls, man harnesses the cellular activity of three aquatic bivalve molluscs. First a spherical nucleus has to be carved from the thick layer of mother-of-pearl in the shell of a freshwater mussel, a unionid. In these shells the calcium carbonate is crystallized as aragonite. The mother- of-pearl, secreted by the mantle, is composed of thousands of parallel layers of tabular crystals. Each layer of aragonite lies on a thin organic film. The grafter cuts out a piece of tissue from the mantle of a marine pearl oyster, places it over the nucleus, and inserts this ensemble, or graft, into the genital gland of another pearl oyster of the same species. The grafted oyster is put back into the sea for several more years, and slowly the cells of the graft cover the nucleus with a layer of mother-of-pearl, or nacre. ln the cultured pearl, this forms layers of aragonite alternating with leaflets of an organic matrix.
However, instead of being arranged flat as in the oyster shell, the layers are positioned concentrically around the nucleus.
The cultured pearl is thus composed entirely of nacre, that of the nucleus being freshwater in origin, while the outer part, marine. Grafters have tried to replace the nacre from Unios by nuclei of marine mother-of-pearl, reconstituted nacre, and even marble, but without satisfactory results. At present all cultured pearl nuclei come from the shells of the freshwater Mississippi mussel, or << pig toe >>, which is threatened by over-exploitation. To succeed in pro- ducing a cultured pearl, two pearl oysters, a donor and a recipient, are needed. To preserve the species, therefore, pearl oyster breeding should be
developped further. In natural or fine pearls, the nucleus is provided by a parasitic intruder, which the oyster isolates first in
a sac formed by the mantle, and then in a pearly envelope. Natural pearls thus rarely have solid nuclei.
In the past, the craze for natural pearls led to the exploitation of millions of oysters. This practice eventually brought about the destruction of most of the natural pearl oyster beds. Natural pearls can still be found in some freshwater pearl mussels. However, these are worth much less. The only natural thing about the imitation pearl, beautiful though it may be, is the coat
of iridescent guanine, obtained from fish scales, with which it is varnished.
Cultured South Sea pearls are of remarkable size. The considerable space inside the larger pearl oysters allows implantation of large nuclei. Moreover the pearl nacre is laid down faster than in the smaller species, giving a thicker final coating around the nucleus. The diameter of these pearls ranges from 9 to 20 mm, while baroque pearls can reach a length of AO mm. The pearl nacre can vary in thickness from 2 to o mm, and it is the quality of the nacre which can give a pearl an exceptional orient and lustre. Pearls from small cold-water pearl oysters, by comparison, rarely succeed in coating the nucleus with more than O2 to 0.6 mm.


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