Pearl is of animal origin and is obtained from Pearl oyster belonging to phylum Mollusca. A pearl has retained its position as a costly ornamental object. The actual nature and mode of formation of pearl inside the oyster has now been explained scientifically. Scientists engaged in making cultured pearls have planned to stimulate this natural phenomenon and succeeded in producing cultured pearls exactly of the same substance as the natural pearl. Making of cultured Pearls which is not a very old venture, is restricted to certain parts of the world.
If a foreign particle accidentally enters between the shell and mantle of the animal then a secretion comes out from the nacre gland or mother-of-pearl of mantle and this secretion surrounds the foreign particle in a concentric manner which later becomes hard and glossy that is known as pearl. Pearl is always formed outside the mantle but inside the shell of mollusk, i.e., it is formed between the shell and mantle of the animal. It is not a mineral product, such as diamond or any other costly mineral stones.
How pearls are formed in nature
Pearl formation is an interesting phenomenon in certain bivalvia of mollusc group of animals. The epithelial cells of the outer layer of mantle which remains just below the shell are responsible for the formation of pearl.
Sometimes a foreign particle accidentally enters between the shell and mantle, of the oyster. These foreign particles are sand grain or parasites or a small insect etc. This foreign body acts as a nucleus and happens to adhere of its mantle. The epithalial layer after the mantle at once encloses the foreign body like a sac. Thus the foreign body is in closed by mantle epithelium and forms a pearl sac. Now, this foreign body acts as an irritant and stimulates epithelial cells of the mantle. Then the epithelial cells of mantle secrete nacre which surrounds this foreign body in the concentric layers. The concentric layers of nacre gradually completely enclose the foreign body from definite point of view. These layers of nacre when harden becomes natural pearl. The size pearl is directly proportional to the degree of irritation which is caused by foreign particle. Time taken for the formation of a pearl of average size is 3—5 years.
1. This type of pearl is formed by natural process.
2. Here nucleus is formed by sand particle, parasite etc.
3. Nucleus enters the body accidentally.
4. Few number of pearls is formed.
5. Growth of pearl occurs in natural environment.
6. They are generally of irregular shape.
1. This type of pearl is produced r. application of technology by hums beings.
2. Nucleus is artificially formed from olluscan shell.
3. Nucleus is introduced by surgical process.
4. Large number of pearls is produce
5. Growth of pearl occurs in human care.
6. They are regular shape as that nucleus.
Process of making cultured pearls:
Japan has the honor of developing techniques to cultured pearls oyster and obtains pearls out of them by introducing artificial devices. These whole devices are very much complicated, technical and time taking. The basic principle of the pearl culture is to introduce some foreign material between shell and mantle of the oyster and out of irritation will produce nacreous layers around the foreign body which in time will become pearl.
Collection of Oysters:
The oysters are collected from the bottom of sea by the experienced divers. Experienced diver possess a small hand net which helps in the collection of oysters from the bottom of sea ; the collected oysters are stocked in the wooden bucket which is attached I chord to the diver's left wrist. Diver goes up to 5 meter depth and can collect 2 oysters per dive. The best time for collection of oysters is from early morning to day in the summer season; because during this period the water is nearer and the remains calm.
Rearing of Oysters:
The collected oysters are first cleaned and then placed into the rearing cages. These cages are divided into few smaller chambers and covered with wire net. These rearing cages are placed in the sea-water as hanging position from the buoy. These cages well protected from natural enemies of oysters, such as Octopus, Eel etc. Rearing c; remain in the sea-water for a period of about 10-20 days ; so that the oysters can recover the strain due to excessive handling and for the adjustment to the shallow water conditions.
Insertion of nucleus:
(a) Fitness of Oyster for operation : The selected oysters for the insertion of nucleus should be healthy and strong, so that they can overcome the shocks during operation.
(b) Preparation of graft tissue : The piece of tissue which is taken from one oyster is transplanted inside the mantle of another is called as graft tissue. A strip of about 7x0.75 cm. is cut from the edge of mantle of healthy oyster by sharp knife. This cut piece is to be cleaned and washed. Now this long strip is again cut into small squares according to the size of the nucleus for insertion. These small squares are kept in sea-water at 22°C for 48 hours. Necre secreting cells of small squares of mantle are found only on the outer surface and so it is essential to keep the outer surface in contact with the inserted nucleus.
(c) Preparation of nucleus: Although any small foreign particle may act as nucleus to initiate the pearl formation but the calcareous nucleus is the best; because the deposition of necre is found to be more satisfactory on calcareous nucleus than other particle. So the nucleus is prepared from the molluscan shell. The shape of the pearl depends on the shape of the nucleus. If the nucleus is spherical, then spherical pearl is formed which is regarded as good quality of cultured pearls.
(d) Insertion of nucleus to oyster : Before the oysters are fixed in a desk clamp, oysters are kept under stress of suffocation. As a result, oysters start to open their shells and a small piece of bamboo peg is inserted between the gap of two shells, so the shells may not be closed again. Now the oysters are fixed in a desk clamp in such a way that the right valve of the oyster is facing upward. Before operation, the oyster is anesthetized by some chemical. After that, the foot of the oyster is carefully exposed and an incision is given on the epithelium of foot. Now one graft tissue piece is placed into the incision place and then the nucleus is placed over the graft tissue. This graft tissue acts as a bed for the nucleus. After that, small bamboo peg is quickly removed. The oyster shells are closed. The whole operation should not go beyond 30 minutes because the oyster cannot survive beyond one hour of the operation period.
(e) Post operational care: After completion of operation, nucleated oysters placed into cages. These cages are suspended into sea-water and the cages are attached with floating rafts. The cages are kept to a depth of 2-3 meters for about 6-7 days..a result, the oysters are able to recover the shocks due to operation. This period of 6 days is regarded as recovery period. After this period, oysters are examined and de oysters if any are removed from the cages. Now a days, living oysters are examined X-rays to see whether the inserted nucleus remains in position or not. About 300 6000 nucleated oysters are kept in different cages. These cages are now suspended sea-water at 2-3 meters depth through floating rafts. These cages are kept general undisturbed in this position for 3-6 years. After which they are ready for harvesting.
Harvesting of cultured pearls from oyster :
Oysters are removed from water after completion of 3 years of insertion of nucleus. These are removed from water in the month of December to February and the pearls are taken out by crushing the oyster. Each oyster can form 3-4 pearls in their 15 years of life span.
Cleaning of cultured pearls after harvesting:
After harvesting of pearls from oysters, the cultured pearls are cleaned properly with soap solution but pearls should not be rubbed much.