The only difference between a cultured pearl and a natural pearl is the human intervention in surgical implantation of a live mantle graft and nucleus for hastening pearl formation to the desired size, shape, colour and lusture. The development of cultured pearls took much of the chance, risk, and guesswork out of the pearl industry, allowing it to become stable and predictable, and fostering its rapid growth over the past 100 years.
Pearls known as ‘Queen of Gems’ have been occupying a unique place due to their fascinating beauty ever since their discovery in ancient times. It is a natural gem and is produced by a living organism, molluscs. With the turn of the last century, the demand for pearls was fast increasing, but their supplies from the existing fishing grounds have diminished due to over exploitation and pollution. A natural pearl is formed when a foreign particle, such as a piece of sand or parasite, makes its way into particular species of mollusc and cannot be expelled. As a defense mechanism, the animal secretes a substance, known as nacre, to coat the foreign body. This simple phenomenon is being exploited in pearl culture practices. Cultured pearls are produced both in marine and freshwater environments. A pearl has structural and chemical properties similar to the pearl nacre of the shell interior. Its shape is determined by the irritant foreign body and its quality by the nature of secretions of the pearl sac. Thus the outer epithelium of the mantle tissue is the “Keynote” in the “Orchestra” of bio-mineralization of a pearl.
A cultured pearl is also a natural pearl, the only difference being the human intervention in surgical implantation of a live mantle graft and nucleus for hastening pearl formation to the desired size, shape, colour and lustre. In India, three species of commonly available freshwater mussels viz., Lamellidens marginalis, L. corrianus and Parreysia corrugata found to produce good quality pearls. Pearls from India are known from time immemorial. Admired all over the world as the finest of ‘Oriental Pearls’ these were in high demand all over the world. While the demand of pearls in India and elsewhere is increasing, their supplies from nature have reduced due to overexploitation and pollution. India is importing a large amount of cultured pearls every year from international market to meet the domestic demand.
Pearl culture technique
Farming practice of the freshwater pearl culture operation involves six major steps sequentially viz., collection of mussels, pre-operative conditioning, surgery, post-operative care, pond culture and harvesting of pearls.
i) Collection of mussels
The healthy mussels are collected from the freshwater bodies like pond, river etc. They are collected manually and kept in buckets or containers with water. The ideal mussel size used for pearl culture is over 8 cm in anterior-posterior length.
ii) Pre-operative conditioning
The collected mussels are kept for pre-operative conditioning for 2 to 3 days by keeping them in crowded condition in captivity with aged tap water at a stocking density of 1 mussel/L. Pre-operative conditioning helps in weakening of adductor muscles, which helps in easy handling during surgery.
iii) Mussel surgery
Depending on the place of surgery the implantation is of three types viz., mantle cavity, mantle tissue and gonadal implantations. The key materials required during the surgical implantations are beads or nuclei, which are usually made from mollusc shell or other calcareous materials.
Mantle cavity implantation: In this procedure round (4-6 mm diameter) or designed (images of Ganesh, Buddha, etc.) beads are inserted into the mantle cavity region of mussel after opening the two valves (without causing injury to mussels at both ends) of animal and separating carefully the mantles of anterior sides from the shell with the help of surgical set. Implantation could be done in mantle cavities of both the valves. In case of implantation of designed beads care is taken such a way that the design portion faces the mantle. After placing the beads in desired place the gaps created during implantation are closed just by pushing the mantle onto the shell.
Mantle tissue implantation: Here the mussels are divided into two groups; the donor and the recipient mussels. The first step in this procedure is preparation of graft (small pieces of mantle tissue). This is done by preparing a mantle ribbon (a strip of mantle along the ventral side of the mussel) from a donor mussel, which is sacrificed, and cutting that into small pieces (2 x 2 mm). The implantation is done on recipient mussels, which is of two types viz., non-nucleated and nucleated. In the former, only the graft pieces are introduced into the pockets created at the inner side of posterior pallial mantle present at the ventral region of the mussel. In the nucleated method, a graft piece followed by a small nucleus (2 mm dia) is introduced in the pockets. In both the procedures care is taken so that graft or nucleus does not come out of the pocket. Implantations could be done at mantle ribbons of both valves.
Gonadal implantation: This procedure also involves preparation of grafts as described earlier (mantle tissue method).First a cut is made at the edge of the gonad of the mussel. Then a graft is inserted into the gonad followed by nucleus (2-4 mm dia) so that the nucleus and graft should be in close contact. Care is taken such a way that nucleus touches the outer epithelial layer of the graft and the intestine is not cut during the surgery.
iv) Post-operative care
Implanted mussels are kept in post-operative care unit in nylon bags for 10 days with antibiotic treatment and supply of natural food. The units are examined daily with removal of dead mussels and the ones that reject the nucleus.
v) Pond culture
After post-operative care the implanted mussels are stocked in the ponds. The mussels are kept in nylon bags (2 mussels per bag) and are hung from bamboo or PVC pipes and placed in ponds at 1 m depth. The mussels are cultured at stocking density of 20,000-30,000/ha. The ponds are fertilised with organic and inorganic fertiliser periodically to sustain the plankton productivity. Periodical checking of mussels with removal of dead ones and cleaning of bags is required throughout the culture period of 12-18 months.
vi) Pearl harvest
At the end of the culture period the mussels are harvested. While individual pearls can be taken out from the mantle tissue or gonad of the live mussels, the mussels are sacrificed in case of mantle cavity method. The products obtained through different surgical implantation methods are shell attached half round and shell attached image pearls in mantle cavity method; unattached small irregular or round pearls in mantle tissue method; and unattached big irregular or round pearls in gonadal method.
Causes of mortalities
Followings are some of the causes of mortality:
- The mussel will die whenever a vital organ is damaged during the operation. The organ may be the intestine, stomach, liver or kidney. Avoid hitting or damaging any organ. Inserting the nucleus in the gonad, near the base of the foot, would result in a lower mortality rate as no vital organ is liable to be damaged.
- Forcing open the mussel shells during the operation can damage the adductor muscles resulting in mortality. Mussels should therefore be opened gently.
- Opening the shells of the mussels too wide can damage the adductor muscles and result in mortality. On an average 7.5 mm shell opening would not result much stress on the mussel.
- Weak or thin mussels should not be operated on as they will most likely not survive the operation and will eventually die. Instead, healthy ones should be selected.
- Mortality will be higher when using larger-sized nuclei than smaller-sized nuclei.
Pearl quality enhancement
To maintain uniformity in colouration and quality, pearls after harvest are subjected to value addition through surface cleaning Or bleaching and dyeing or both which may also enhance their value. Traditionally, surface cleaning or bleaching is done using hydrogen peroxide, ether solvents, water and alcohol in different concentrations depending on the requirement. Attempt have also been made to find out other pearl surface cleaning and bleaching agents using the physical procedure such as ultrasonications and chemical treatment using (i) EDTA (ii) Sodium hypochiorite (NaOCl) and (iii) Bleaching powder. The ultrasonic action causes the cleaning fluid (the traditional herbal cleaning agents like Sikakai and Rithafal juice) to be forced into the surface to be cleaned and resulted complete removal of any adhered particles. The penetrating effect of the ultrasonic vibration enables the cleaning solvent to be forced into inaccessible places. EDTA is considered as a surface cleaning agent as it is known to be a chelating agent for organic matter impurities. Sodium hypochlorite (NaOCl) and bleaching powder are also considered to be potential cleaning agents as they act as oxidizing and bleaching agents respectively. It is found that EDTA, NaOCl, and Chlorine along with hydrogen peroxide have indicated utility in imparting shine to the cultured pearls.
Identification of real pearl
Irregularities: Minor irregularities in colour are an indication that the pearl are genuine. One of the more telltale signs of imitation pearls is finding black pearls that are large, symmetrical, and perfectly matched in every way. A strand such as this does not exist as real pearls, since all pearls are slightly different.
Examination of drill hole: Very small drill hole may indicate natural pearl. By looking at the drill hole carefully with a jeweler’s magnifying glass, the line of demarcation between the mother of pearl nucleus and layer of conchiolin may be visible appearing as a darker line. When the darker line is visible the pearl is cultured pearl. Splitting of nacre material around the nucleus indicates it as artificial pearl.
Tooth Test: A common and fairly reliable test for fake pearls is to simply rub the pearl gently across your teeth. The surface of real pearl should feel rough or “sandy” due to the unique build up of the nacreous surface. A fake or imitation pearl will generally feel smooth across your teeth, such as glass or plastic.
Luster test: Cultured pearls will have a depth of luster whereas imitation pearls usually have a surface shine but no inner glow. The difference can be seen by comparing both side by side.
In contrast to natural pearls, which have widely varying shapes, sizes, and qualities, and which are difficult to find, cultured pearls could be “designed” from the start to be round and primarily flawless. The mussels could be monitored until each pearl was fully formed, thus better ensuring their health and survival and the pearls could be produced, thereby bringing their cost down to a point where pearls became accessible to large numbers of people. In short, the development of cultured pearls took much of the chance, risk, and guesswork out of the pearl industry, allowing it to become stable and predictable, and fostering its rapid growth over the past 100 years. Today more than 99% of all pearls sold worldwide are cultured pearls.
(Disclaimer: The author is working in the Department of Aquaculture, College of Fisheries, Guru Angad Dev Veterinary and Animal Sciences University (GADVASU), Ludhiana. Views expressed are personal. He can be reached at email@example.com.)