Lombok Pearls : Cultured Pearls General Rule & Exceptions

Figure 9. Schematic diagram explaining the general way of cultured pearl production in the two organs where a saibo transplant is successful in respect of pearl formation. Modified after Schöffel, 1996. — Figure 9. Schematic diagram explaining the general way of cultured pearl production in the two organs where a
saibo transplant is successful in respect of pearl formation. Modified after Schöffel, 1996.

Gonad-grown cultured pearls The gonad (reproduction organ) accepts transplanted mantle tissue graft (Japanese ‘saibo’) that will grow out to a pearl sac. As gonad cells cannot precipitate CaCO3 the introduction of the saibo is mandatory. The gonad is deep in the shell where the two shell halves are at an important distance from each other. Here, the space for one or more beads is provided. Nuclei are brought into the gonad and a close contact to the saibo. The process of grafting is often named ‘nucleation’, a term that is also used for inserting the bead nucleus. For clarity reasons it would therefore be better to differentiate and use both terms, ‘grafting’ and ‘beading’. (Reference: Lombok Pearls)

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The introduction of a bead is optional and can be made more than once. The grafting is only done once, to start the formation of a pearl sac. All gonad-grown pearls are cultured, as only transplanted mantle tissue cells can form nacre. A general method to produce gonad grown cultured pearls containing a spherical or other shape nucleus has furnished the well-known Akoya (Pinctada martensii), South Sea Pearls (Pinctada maxima) or lombok pearls and Tahiti cultured pearls (Pinctada margaritifera) (Müller, 1997) (Reference: Lombok Pearls).

Black pearls are also produced in the Sea of Cortez (Pteria sterna) (Kiefert et al., 2004) (Reference: Lombok Pearls) . Since recently, small akoya-type cultured pearls have been grown in the Persian Gulf in Pinctada radiata shells. Figure 9 gives a schematic view on the general methods of producing cultured pearls in the gonad.

Figure 10. Schematic diagram explaining the modifications from the general way of cultured pearl production. These variations lead to more recent products now available in the trade. Modified after Schöffel, 1996.

In Figure 10 the variations of the normal routines are shown. It is worth mentioning that most pearl farms are very careful and precise when they select tissue donor oysters that will be sacrificed for their mantle tissue. Only individuals with outstanding nacre quality will be taken for producing mantle tissue pieces, as this quality will vastly define the quality of the resulting pearl. Recipient oysters, on the other hand, must not have nice nacre, but must be healthy, fast growing and provide good housing for the bead and saibo.

Quite often the contact between the bead and the saibo is not very close, due to negligent manipulation by the transplanting technician. Such pearls will be drop shaped or have an extension on the side where the saibo was placed. Should the saibo not reach the bead, the latter will be rejected and the pearl grows as beadless. Due to the missing pre-form (spherical) the pearl will end as a more or less round baroque shaped pebble.

The trade uses the term ‘keshi’ for these products. A more precise and descriptive name would be appropriate, such as non-beaded or beadless cultured pearl of baroque shape. This applies for both: South Sea ‘Keshi’ and Tahiti ‘Keshi’ (Hänni, 2007 a, b). (Reference: Lombok Pearls)

The term ‘keshi’ comes from the Japanese pearl culturing tradition and is used for very small mantle-grown pearls, by-products of the Akoya production. They form as a consequence of injuries on the rim of the shell, due to rough handling when transported and operated. That they are about 2 mm is explained by the short time between injury and harvest. That large gonad-grown South Sea Keshis formed after a tissue transplant have nothing to do with
those minute mantle-grown Japanese Keshis without saibo graft seems obvious.

When some years ago a dealer reported, ‘they are now growing keshis with beads’ it became evident how important it is, to use proper naming. What the dealer was describing were gonad-grown cultured pearls with baroque shaped beads (Figure 11) (Reference: Lombok Pearls). Flat baroque freshwater pearls from China are used as nuclei and overgrown with Pinctada margaritifera dark nacre, producing more baroque pearls.

Figure 11. Beadless freshwater cultured pearls from China with a flat irregular shape are used as beads for producing baroque P. margaritifera pearls. Upper samples are shown as grown, lower samples are cut open and polished; they reveal the core of freshwater nacre. Photo © H.A. Hänni, SSEF and GemExpert.

Figure 12. Four natural pearls with different shape and appearance were used as beads in P.maxima oysters. Upper row shows the coated pearls, middle row shows x-ray pictures, and lower row shows 3 cross sections. Photo © H.A. Hänni, SSEF and GemExpert.

An experiment
In previous years the author has used spherical Chinese freshwater cultured pearls as bead material in Pinctada maxima or lombok pearls and Pinctada margaritifera oysters. The procedure and the results have been reported recently (Hänni et al., 2010 a, b). It was then thus worth trying to use poor looking natural pearls as bead nuclei and provide them with a nice P.radiata coating. The experiment was carried out in the Gulf region where local P. radiata shells have been producing the famous Oriental natural pearls from the Gulf region for hundreds of years (see again Figure 2). (Reference: Lombok Pearls)

In May 2010, a number of natural pearls of different sizes and shapes were seeded with a mantle tissue graft into the gonad of 9 cm P. radiata shells. Three months later a sample harvest was done in order to measure the coating and to count the number of aragonite platelets formed in that period of time. A further sampling was done in November 2010 and tests were carried out, including the x-ray recording of some samples (Figure 12). (Reference: Lombok Pearls)

Previous to this natural pearl coating experiment, natural pearls of brown colour and columnar structure were seeded in P. maxima oysters. The pearls probably are Pinna pearls, their structure corresponds to ‘unripe’ natural pearls. This means that the shells were harvested too early, as they consisted only of the columnar core but had not yet been overgrown by nacre (compare with Figure 1). Results of coated natural pearls by P. maxima oysters, in the shell for 16 months, are shown in Figure 13. (Reference: Lombok Pearls)

Figure 13. A selection of pearls with natural non-nacreous (‘unripe’) pearls used as nuclei, with a coating of P. maxima nacre. Radiograph shows darker concentric rings as characteristic for natural pearls. Photo © H.A. Hänni, SSEF and GemExpert.

Any object fitting in size and having an inert character can be coated with nacre, even a small marine snail shell. To demonstrate the possibility that any core of appropriate size is easily coated with nacre once it is implanted with a saibo, an experiment with trilobites, a fossil of Cambrian age (approx. 500 my) was carried out using P. maxima or lombok pearls oysters (Figure 14). (Reference: Lombok Pearls)

Figure 14. Trilobites from North Africa were used as cores and became overgrown by nacre in a pearl culturing process with P. maxima. Above, some trilobites of approx. 5 mm size. Below, the nacrecoated bodies after a 9-month stay in a P. maxima. Photo © H.A. Hänni, SSEF and GemExpert.

Conclusions

Natural pearls are the reaction of the mantle on an injury caused to the juvenile mantle. External mantle cells displaced to a deeper layer (conjunctive tissue) grow out and constitute a pearl sac that will accumulate CaCO3 and form the natural pearl. Mantle-keshis reported in Japanese Akoya shells are evidence of this process. The injuries occurred during rough handling of the shells in the farms where the rim was damaged. Cultured pearls or lombok pearls are either grown in wild shells collected from the sea, nursed wild spat or from shells grown in hatcheries with a brood stock of selected characteristics. Freshwater mussels are raised in basins where specific fish act as hosts for the larvae. These domestic bivalves are then subjected to a surgical operation where mantle tissue pieces (saibo) are grafted into either the gonad or into the mantle.

Individuals with high quality nacre are selected as tissue donors as such nacre will form the present cultured pearl. Recipient oysters have to be strong, fast growing and resistant to infections. Mantle and gonad are the two organs of an oyster or mussel where a tissue graft can survive and produce CaCO3. Once grafted, the saibo will grow out to a pearl sac. It is optional to add a bead to the saibo graft. Shape and size of a bead nucleus depend on size of shell and where it will be implanted. Generally round beads are placed into the gonad to produce gonad-grown pearls. Coin shaped beads are put into the mantle, and later replaced by spherical ones, when more space is available. Once a pearl sac is formed, it can be used a second time. Re-beading is performed when a first pearl shows a good quality.

There is a certain variety in bead material used (Superchi et al., 2008). (Reference: Lombok Pearls). Traditionally, beads are made of freshwater shell material and according to the size a small number of mussels are used. Washboard mussels may have thick walls and produce beads up to 20 mm. Large composed beads may also be cut from saltwater shell (e.g. P. maxima or lombok pearls) when shell pieces are ground flat and glued together forming a laminate.

Besides common bead material, almost anything that fits in size and is not spiky will get coated with nacre. Experiments with fossils and even natural pearls have shown positive results. Pearl sacs in the mantle of Chinese freshwater mussel, after harvesting a coated coin nucleus, can be filled with mud (G. Wiesauer, pers. comm. 2012). The result is a baroque mantle-grown pearl, now available on the market.

Acknowledgements

Thanks go to the pearl farmers who invited the author to perform interesting experiments in their pearling enterprises. Andy Müller (Hinata Trading, Kobe) has continuously furnished new cultured pearls that attracted our interest. To Mr René Hodel (Hodel of Switzerland, Hong Kong) the author owes thanks for an important literature reference. Georg Wiesauer has been updating the author on freshwater cultured pearls from China. Dr. Michael Krzemnicki, Dr. Franz Herzog, SSEF Swiss Gemmological Institute and Laurent Cartier, University of Basel, (Basel, Switzerland) have supported our research by analytical assistance and always been active partners in discussing the topic. Laurent Cartier has further reviewed the English and proposed significant improvements of the original paper. (Reference: Lombok Pearls)

Articles source: Natural pearls and cultured pearls: A basic concept and its variations, Prof. Dr H.A. Hänni, The Australian Gemmologist | Third Quarter 2012 | Volume 24, Number 11 – (Reference: Lombok Pearls)

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