Date of Issue : 5 May 2011
The Post of Northern Cook Islands recently issued a wonderful set of stamps and souvenir sheet on pearl industry on 5th May 2011.
Formation of Pearl
A pearl is formed when the mantle tissue is injured by a parasite, an attack of a fish or another event that damages the external fragile rim of the shell of a mollusk shell bivalve or gastropod. In response, the mantle tissue of the mollusk secretes nacre into the pearl sac, a cyst that forms during the healing process. Chemically speaking, this is calcium carbonate and a fibrous protein called conchiolin. As the nacre builds up in layers of minute aragonite tablets, it fills the growing pearl sac and eventually forms a pearl. It is a myth that a grain of sand can cause a pearl to form, as nacre will not adhere to inorganic substances.
Natural pearls are those pearls that are formed in nature, more or less by chance. Cultured pearls, by contrast, are those in which humans take a helping hand. By actually inserting a tissue graft of a donor oyster, a pearl sac forms, and its inner side precipitates calcium carbonate in the form of nacre.
Two species of pearl oyster are found in the Cook Islands
• The small pearl oyster or pipi (Pinctada maculata) is widespread throughout the Cook Islands. On Penrhyn the oyster is harvested for its natural golden pipi pearl.
• The black-lipped pearl oyster or parau (Pinctada margaritifera) is abundant in the Manihiki and Penrhyn lagoons. In the wild they are found attached to coral reefs in depths of 5 to 60 metres. In the past, divers collected the pearl oysters, and the shell (mother-of-pearl) were exported to be made into products such as buttons. However, these days, black-lipped oysters are more valuable if kept alive and cultured for their black pearls.
Farming pearl oysters requires drilling the oysters and hanging them on ropes set in the lagoon. The cultured black pearl actually begins as a small bead inserted or ‘seeded’ into the oyster. It takes about two years before the bead is coated as a black pearl. Many factors affect the success rate, but of 100 oysters seeded, typically only five will produce a high-quality round pearl.
There are important environmental factors to consider when pearl farming. Because pearl oysters filter up to 20 litres of water per hour, they must be kept well spaced apart to ensure there is an adequate supply of food in the water. If not, the oysters become starved and growth is poor, which may cause oyster disease.
Presently, there are about 110 pearl farms on Manihiki and 1.5 million adult oysters being cultured. On Penrhyn there are about 100 pearl farms and about 200,000 cultured oysters.
From Our Readers….
Mr Sanjiv Jain of Dehradun shares here a private Maxim Card on recently issued stamp of Dr DS Kothari .
