Sunday, 25 May 2008

How a pearl is formed

Not all mother-of-pearl, of course, is derived from oysters. Of more than 8,000 species of mollusc less than 20 are considered reliable sources for pearls. Some are edible but usually it is flesh or shell. Most molluscs produce nacre – a mix of calcium carbonate and conchiolin – to create their shells, even the humble mussel (which in Mississippi would be turned into shirt buttons right up to the last century) but more usually for workable mother-of-pearl it is abalone oysters, green, paua or red abalone, the silver or gold lipped oyster, the ayoka oyster, or the Ceylon oyster (pictured), all of which secrete more copious and valued fluid. The nacre forms to fight off an invading irritant such as a parasite or a food-particle by smothering it.

The value of a pearl is determined by the finesse and regularity of the crystals formed in this process. The number of layers formed will give the orb lustre and iridescence.

In warm waters – around 30 C - as in the Pacific, the oyster metabolism increases and they grow faster and secrete more nacre, but the layers are thick and not as translucent and the crystal structure is not perfect resulting in a duller, less lustrous mother-of-pearl. So the great oyster eating areas like the Chesapeake Bay in north America have barely ever seen a pearl formed, but further south in the Mississippi huge caches have been found in ancient burial tombs.

Ideally, when temperatures are lower – around 16C - the oyster metabolism is slower, and it produces the nacre more slowly. These nacre layers are thinner and the crystal structure more even, resulting in an increased translucency and higher value. Even now, dealers will test a pearl by rolling it on their tongue to see if it is real or artificial – artificial pearls are completely round.

The colours of the pearl derive from the shades of plankton that the oysters filter in the water and so in a way are a photocopy of the micro plankton on which they feed.

Hence the pearls of the south seas are black or green. The crystal is orthorhombic, meaning it has three triangular sides, which act as tiny prisms. It is the interaction of light with these tiny prisms in the pearl that create the quality referred to as orient.

The prized south sea oyster Pinctada margaritefera can secret three or four layers of aragonite each day. Over a two-year life cycle, some 2,000 layers will have been deposited, each one about one micron or 0.001 mm thick.

The Italian director Federico Fellini commented aptly: “All art is autobiographical; the pearl is the oyster's autobiography”. You could also say the pearl is also mankind biography.