Cultured pearls are produced by pearl oysters as a result of delicate techniques developed by scientists over many years. However, the human role in the pearl production process stops with the successful completion of surgery for nucleus implantation and in providing suitable environmental conditions through careful selection of the culture sites. The actual pearl formation, which results due to an internal biological process, is left entirely to the pearl oyster itself. The quality of pearls with regard to their shape and colour, which are important factors in terms of their market value, are not only influenced by external environmental factors, but also by the inherited capabilities of individual oysters.
Free spherical cultured pearls are produced with the help of spherical shell beads, known as nuclei. These beads are manufactured from the shell of a freshwater mussel species found in the Tennessee and Mississippi Rivers in the U.S.A. These shells are collected and exported to Japan, where they are cut and processed into precisely spherical beads of different diameter.
In the initial phase of the development of the implantation technique, the shell bead nuclei were wrapped in mantle tissue and implanted into the visceral mass of the pearl oyster. Later it was found that a small piece of the mantle epithelium would suffice for the formation of the pearl-sac, which is responsible for the secretion and deposition of the mother of pearl on the nucleus itself. A small piece of mantle section measuring 2×3 mm in size, is sufficient for nuclei of 2–6 mm in diameter. The thickness of the epithelium layer plays an important part in the formation and quality of the pearls. The inner epithelium and the connective tissue rapidly disintegrate, leaving only the outer epithelium layer to proliferate and form the pearl-sac. Water soluble eosin and mercurochrome are some of the chemicals used as sterilising and colouring agents to enable the operator to place correctly the graft tissue, in terms of its orientation, at the time of insertion.
Oysters may die in the culture farm due to a variety of reasons. A common cause of death is serious infection of the wounds inflicted at the time of the implantation operation. However, diseases, biofouling, shell boring and pollution may also be responsible for oyster mortality. Proper farm management procedures usually keeps the oyster mortality rate below 10 %.
Nuclei ejection does not commonly occur, particularly in the case of large ones. However, this miss-happening can be kept to the minimum by improved surgical methods. Discharged nuclei are usually thrown away. However, in some cases they are retained for re-implantation as no nacre has been deposited over them, probably due to the non-formation of the pearl-sac. Defects in surgery and incorrect orientation of the nucleus and graft are two of the reasons for this defect. In cases of non-formation of pearls, the nucleus may either remain as it is or become eroded.
As in the case of natural pearls, the production of cultured pearls is also a result of a biological process within the pearl oyster itself. Therefore, the quality and the rate of cultured pearl production are only partly controlled by the activities of the pearl culturists. The production rate can be improved by adopting appropriate technology and care during surgery and subsequent culture. ‘Gross production’ usually refers to the number of cultured pearls produced by oysters which have survived the implementation of the shell bead. In India, the highest recorded production rate achieved in single implantation was about 65%, while in multiple implantation, a rate up to 180% has been obtained. This rate can definitely be improved by professional technicians with considerable knowledge and practical experience.
The gross production of pearls includes all kinds of pearls, from the finest quality to trash. Usually a small percentage of the pearls produced have an outstanding colouration and perfectly round shape, while a large proportion are inferior and some totally valueless as gems. This is a common feature in all pearl culture centres around the world. Generally the success of a pearl culture industry depends greatly on a high production of good quality pearls as a percentage of the total numbers produced.
Shirai (1970) has categorised the cultured pearls as follows:
Features: flawless, one flaw, small flaws, small stain marks, pink, silver or light cream in colouration. These pearls are further classified into:
|A-1:||Top pearls-perfectly round, pink, flawless and lustrous. This class may also include pearls with small blemishes of the size of a pin point.|
|A-2:||First class pearls - with slightly larger pits and protuberances. When treated these pearls become indistinguishable from the pearls categorized as A-1.|
Features: fairly large flaws, stain marks, creamy in colouration, and irregularities in the shape.
Features: wild shaped, badly coated, heavily marked, clayey lumps, half good and half bad. These pearls are usually referred to as ‘trash pearls’.
Generally, in a commercial pearl culture farm the combination of class A and class B pearls should account for at least 60 % of the gross production of cultured pearls for it to run economically. The remaining 40 % of class C pearls are usually rejected. However, their nuclei can be often salvaged and reprocessed.
Harvesting of the cultured pearls is usually carried out manually. However, the process can be automated with the use of simple machines. In the case of manual extraction the pearls are collected by initially separating the two shell valves, by cutting the adductor muscle, making an incision on the gonad and squeezing the pearl out. The machines used for pearl extraction usually work by dissolving the oyster soft body parts with the use of chemicals while the pearls remain as they are and become easily extractable. In case the oysters need to be re-used for a second time, the pearls are carefully removed by opening the pearl-sac through the gonad taking care not to damage nor stress the oyster. The oysters are then returned to the culture site for recovery, and after a certain length of time they can be operated for a second time to produce additional pearls. In Japan, harvesting is done during periods of low temperature and pH as the pearls tend to be of higher quality due to a thick and compact outer nacreous layer.
Finally, the harvested pearls are washed in distilled water, polished with refined salt and sorted for sale according to size, colour, shape, lustre and other external characteristics.