Quality commands a premium price in pearls. The success of the pearl culture industry depends on the high rate of production of quality pearls. Therefore, considerable attention is paid to this aspect. The value of a pearl is decided by its quality, size, shape, colour and lustre. Exceptional pearls command special premium price.
Being a product of biological origin, individual variations are bound to occur in each and every pearl. The secretion of the mantle or the pearl-sac, which leads to the formation of the pearl, may be organic or inorganic in origin, or a combination of both, with unpredictable and subtle variations in structure and composition. The final product may range from the finest to the trash due to such variations.
Even under the highest possible human control, the quality of cultured pearls cannot be controlled absolutely, but can be considerably improved through appropriate care in surgery and during farming. Only the size and shape of the cultured pearls are under the control of the pearl culturist. However, the colour and lustre, which depend on the secretion of the pearlsac, can also be improved to some extent by proper understanding of individual biological and physiological factors as well as the environmental conditions of the culture farms, which influence the formation of pearls.
To achieve a high rate of production of quality pearls, the following factors are required to be taken care of:
Large oysters, in terms of size and weight should be selected. They must be free from a heavy fouling load and blisters caused by sponges and polychaetes. The oysters should be healthy as can be judged from the colour of the visceral mass and gills.
The amount of menthol required to narcotize, and the duration of narcotization should be carefully adjusted depending on the volume and weight of the oysters.
The graft tissue is one of the most critical factors in controlling the rate of pearl production. The donor oyster should be of the desirable size with a well developed and healthy mantle. Extreme care should be taken in selecting, stretching, cleaning, trimming and cutting of the donor mantle tissue. Good water quality and correct level of the chemical agents should be used in maintaining the tissue pieces.
The nucleus implantation is one of the most important factors in cultured pearl production. Its success greatly depends on the selection of the correct site and skill of the technician. The positioning and orientation of graft tissue in contact with nucleus is also critical and should be carried out with great skill and patience. Multiple nucleus implantation requires still greater care and patience.
Oysters can be made to recover from the effect of narcotization through periodic changes of water or gentle flow-through. Sufficient time must be allowed for the incision wound to heal before taking the oysters to the sea for further farming.
The tools must be sharp, rust-free and should have been either sterilized or suitably cleaned and sun-dried.
Different molluscs produce pearls of different colours. The colour of a pearl is usually similar to the colour of the shell nacre of the mollusc which produces it; this character is genetically controlled. This is very clearly shown by Pinctada margaritifera (black or steel grey), P. maxima (silvery white), abalones (green) and freshwater mussels (pink). However, in the case of P. fucata, the colour of the pearls produced may be golden yellow, pink, white or cream, depending on slight differences in the site of nuclei implantation. The pearls produced in the ventral region of the gonad are white or golden, while those produced in the dorsal region of the gonad, in proximity to the hepato-pancreas, are usually grey or white. Flawless pearls of regular form are frequently seen among the pearls developed in contact with internal organs, such as liver, byssal gland and intestine. Pearls produced close to the retractor muscle tend to be baroque in shape with irregular protrusions and with a distinct black colouration.
The thickness of the epithelium of the graft tissue is also considered to be responsible in determining the quality of pearls. A thin graft (2–10 μm) usually produces pearls with a good surface, while thicker ones (>20 μm) tend to produce dull and badly coloured pearls.
A number of environmental factors plays a predominant role in determining the colour and lustre of the pearl nacre. Water depth is one of the most important factors, as quality pearls tend to be produced in waters below 10 m. Fouling and boring problems and siltation are considerably less at depths of 10 m or more.
The pearl culture grounds also play a significant role in determining pearl quality, and repeated culture on the same ground has been shown to affect the quality of pearls. Organic substances discharged by the pearl oysters and fouling organisms are deposited on the sea bottom and their build-up eventually affects the chemical and physical state of the water. Periodic removal of these deposits increases the production of pearls with desirable quality. During the final “make-up culture” period, pearl oysters are shifted to places of potential quality pearl yielding grounds.
Temperature controls the metabolic rate of the molluscs. Higher temperature leads to faster growth in oysters and higher rate of nacre deposition. But this affects the quality of pearls. Thinner laminar nacreous layers, which result from low temperature and pH, are desirable at least in the later phase of the culture period, since the thinner mineral laminae in the upper layers of the pearl give a better lustre to the pearls.
The physiological state of the pearl oyster and the condition of the culture ground have bearing on oyster growth and the size and colour of pearl. This depends principally on differences in chemical composition of the seawater, as well as the kind and amount of plankton in the area where the pearl oysters are reared. The chief source of conchiolin are the nitrogenous substances of the plankton, which influences the colour of pearls.
Minerals and trace elements in the seawater are important, as these also influence the colour of pearls. It has been found that the golden and cream coloured pearls contain more copper and silver, while skin coloured and pink pearls contain more sodium and zinc. The golden coloured pearls have been found to contain more metallic elements than green pearls. The pearl colour varies according to the amount of porphyrins and metalloporphyrins present in them. Iron-bound peptide in the nacre favours the formation of yellow pearls. The organic substances deposited at the beginning of the pearl formation also would influence the colour. Good quality blue pearls are of this origin.
Short duration culture practice is another cause for inferior quality of pearls. Pearls should be allowed to reach maturity in proper time. Pearls with 0.5 mm nacre are accepted in the market.
The rate of nacre growth is dependent on the size of the nucleus. The pearl growth rate obtained at Tuticorin is as follows:
|Nucleus diameter (mm)||Thickness of nacre (mm)||Duration of culture (days)|
The rate of pearl growth in relation to the size of oysters, as obtained at Tuticorin, is as follows:
|Oyster size (mm)||Nucleus diameter (mm)||Thickness of nacre (mm)||Duration of culture (days)|
The quality of mantle of the donor oyster also influences the quality of the pearls. Utmost care should be taken in the selection of donor oysters and in the process of graft tissue preparation.
Processing of cultured pearls through bleaching and dyeing is a highly specialised technique for value addition. It is managed by the pearl processing technicians according to the market needs of various trading centres. It is believed that most of the cultured pearls in the market go through some kind of processing to remove minor defects and improve colour.
The structure and composition of pearls reveal that several formations are possible during the development of cultured pearls. Only those formed by aragonite crystals in tabular form, presenting a regular laminar brick wall-like structure with micro-layers of elemental mineral lamellae alternating with homogeneously deposited organic matrix in concentric layers around the inserted nucleus, would qualify as gems. The rest of the formations will have less or no commercial value.
The lustre of the pearl is due to the play of light on the laminated aragonite layers of the nacre due to absorption and reflection of the waves of incident light. Homogeneity, thinness and smoothness of these layers are responsible for the great play of lustre. Therefore, it is very important that the final phase of post-operation culture of every batch of oysters should be done under ideal conditions.
Cultured pearls are broadly classified into:
These are determined by the quality of secretion of the pearlsac. Nacreous layer pearls are composed of aragonite crystals of calcium carbonate and they alone are valued as gems. The prismatic layer pearls are formed by calcite crystals and the organic ones by proteinous layers of conchiolin.