Pearls are formed from pearl oysters. They are part of a group of animals belonging to the group bivalvia. This includes snails, slugs etc. Pearls are grown in live oysters far below the surface of the sea. These oysters have a hard shell covering their body. In between the shell and its body there is an empty space called the mantle.
A natural pearl begins its life as a foreign object, such as a parasite or piece of shell that accidentally lodges itself inside the mantle from where it cannot be expelled. It causes some irritation to the oyster. To ease this irritant, the oyster's body begins to secrete a smooth, hard crystalline substance around the irritant.
This substance is called `nacre'. As long as the irritant remains within its body, the oyster will continue to secrete nacre around it, layer upon layer. Over time, the irritant will be completely encased by the silky crystalline coatings. And the result is a lustrous pearl.
Nacre is composed of microscopic crystals of calcium carbonate, aligned perfectly with one another, so that light passing along the axis of one crystal is reflected and refracted by another to produce a rainbow of light and colour.
Pearls can be cultured from oysters artificially in an almost identical fashion. The only difference is that a person carefully implants the irritant in the oyster, rather than leaving it to chance. Actually we can isolate strains of oysters that possess superior pearl-producing qualities.
In a process called nucleation or grafting or seeding, skilled technicians carefully open live pearl oysters, and with surgical precision make an incision in its body. They place a tiny piece of `mantle tissue' from another oyster into a relatively safe location. Then, they place a small round piece of shell, or `nucleus,' beside the inserted mantle tissue.
The nucleus is a mother-of-pearl bead made from freshwater mussel. The cells from the mantle tissue develop around the nucleus forming a sac, which closes and starts to secrete nacre, the crystalline substance that forms the pearl.
The nucleated oysters are then returned to the sea where, in sheltered bays rich in nutrients, they feed and grow, depositing layer after layer of lustrous nacre around the nuclei implanted within them.
The oysters are given the utmost care during this time, while suspended in the water, from the rafts above. Technicians check water temperatures and feeding conditions daily at various depths, moving the oysters up or down as appropriate.
Periodically, the oysters are lifted from the sea for cleaning and health treatments. Seaweed, barnacles and other sea borne organisms that might interfere with their feeding are removed from the oysters' shells.
The shells are also treated with medicinal compounds to discourage parasites. After months of care oysters are ready for harvest. Those that have survived are brought ashore and pearls are harvested.
Saltwater cultured pearls can never be mass-produced. Millions of oysters are nucleated every year, but only a small proportion live to produce fine-quality pearls. Many oysters don't survive nucleation, others are weak and fall prey to disease.
Source : The Hindu