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Keshi pearls wholesale : Natural pearls and cultured pearls (A basic concept and its variations)

Keshi pearls wholesale : Natural pearls and cultured pearls – A basic concept and its variations

keshi pearls wholesaleAbstract: For the production of cultured pearls a small number of main options can be chosen to constitute a general method. Non-beaded cultured pearls are usually mantle-grown in freshwater mussels. Beaded cultured pearls are usually gonad grown in saltwater oysters. Minor variations lead to a greater number of different products available in the market today. With this article the author intends to remind the basic concept and possible variations in order to use correct terms for the different products. (source: Keshi pearls wholesale)

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Introduction

In previous articles one of the authors has described the principles of growing cultured pearls and the usage of various bead materials (Hänni, 1997; Hänni, 1999; Hänni, 2006; Strack, 2006; Southgate and Lucas, 2008; Superchi et al., 2008; Hänni et al., 2010 a, b; Hänni, 2011). (source: Keshi pearls wholesale)

From former investigations it became obvious that a large variety of nucleus materials and shapes can be coated with nacre together with the inserting of a piece of mantle tissue grafted into the mantle or the gonad. A core introduction process is optional and can be performed in saltwater pearl oyster or freshwater mussels. The options for making cultured pearls can be summarized in Table 1. (source: Keshi pearls wholesale)

The terms used for a product description have to be in line with the nomenclature regarding pearls (natural, cultured and imitations), and must always be in line with the international standards and trade rules of CIBJO – The World Jewellery Confederation (CIBJO, 2007). (source: Keshi pearls wholesale)

Figure 1. Schematic diagram of an opened oyster. The outer part of the shell is made of columnar calcite (1), the inner part consists of nacre (2). The mantle is retracted on the diagram and extends to the outset part of rim in original state. Modified after Schöffel, 1996.
Figure 1. Schematic diagram of an opened oyster. The outer part of the shell is made of columnar calcite (1), the inner part consists of nacre (2). The mantle is retracted on the diagram and extends to the outset part of rim in original state. Modified after Schöffel, 1996.

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Capacity of mantle tissue

The mantle is a part of the body of the shell. The mantle lines both wings of the shell, and the outer layer of mantle epithelium cells have the capacity to secrete calcium carbonate (CaCO3) in different mineral varieties and geometric shapes, characteristic for each species. Shells of gastropods and bivalves consist mainly of aragonite in tabular or fibrous array. The first product secreted by the very young mantle tissue cells is an organic thin layer of conchiolin. (source: Keshi pearls wholesale)

Right after that the same cells follow the genetic programme and produce CaCO3 with subordinate amounts of conchiolin. While younger mantle cells work on forming calcite in parallel prismatic orientation, older mantle cells lay the bricks that constitute nacre: the aragonite tablets. Figure 1 gives a survey of an oyster and its organs in respect to shell and pearl growth. (source: Keshi pearls wholesale)

Evidence of these two tasks of the mantle tissue cells is furnished when one looks at an open shell, e.g. Pinctada radiata. Its outer rim is of brown colour and is not shiny (Figure 2). (source: Keshi pearls wholesale)

Figure 2. A shell of Pinctada radiata from Bahrain with a natural pearl (6 mm). The organs have been removed and the inner shell shows the two growth sections: columnar calcite (brown) and mother of pearl (silvery with iridescence colours). Photo © H.A. Hänni, SSEF and GemExpert.
Figure 2. A shell of Pinctada radiata from Bahrain with a natural pearl (6 mm). The organs have been removed and the inner shell shows the two growth sections: columnar calcite (brown) and mother of pearl (silvery with iridescence colours). Photo © H.A. Hänni, SSEF and GemExpert.

It represents the mantle’s first calcium carbonate formation, the columnar calcite part. Subsequent to the brown rim we recognise the silvery white part that corresponds to the product precipitated by older mantle cells: aragonite tablets i.e. nacre, mother of pearl. The mantle tissue cells have the know-how of nacre formation. A natural pearl may show the same products from the centre to the outside: first columnar calcite, then a coating of aragonite nacre (Figure 3). (source: Keshi pearls wholesale)

Figure 3. Cross-section through a drilled natural pearl of approximate diameter of 5 mm. The inner part is rich in organic material and shows a columnar structure, made of calcite prisms. The outer part shows fine concentric rings and is made of nacre, aragonite in sub-microscopic tablets. Photo © H.A. Hänni, SSEF and GemExpert.
Figure 3. Cross-section through a drilled natural pearl of approximate diameter of 5 mm. The inner part is rich in organic material and shows a columnar structure, made of calcite prisms. The outer part shows fine concentric rings and is made of nacre, aragonite in sub-microscopic tablets. Photo © H.A. Hänni, SSEF and GemExpert.

The conchiolin is too thin and only seen by the dark colour that it lends the columnar growth area. This succession directs our explanation of natural pearl formation to the juvenile mantle tissue, producing brown columnar calcite before nacre. (source: Keshi pearls wholesale)

Natural and cultured blisters
Bodies placed under the mantle of the shell become coated during the normal precipitation of nacre to increase the shell thickness. Even a dead fish could serve as core for the blister formation as long as the shell is kept horizontal in the net (container in the pearl farm) for a certain period of time. In this orientation the oyster cannot move and get rid of the foreign body. Bari and Lam (2009, pp. 26-27) describe a fish buried under nacre layers as natural formation.

It is however easy to slide a dead fish or a crab unde(source: Keshi pearls wholesale)r the mantle of an oyster, and such blisters are not at all rare or natural (Figure 5). (source: Keshi pearls wholesale)

Figure 5. A dead symbiotic oyster crab (left: down side, right: upside) was buried under the mantle and then subsequently covered with nacre. In nature, the shell would have expelled the crab by just flushing it out. As the P. maxima oyster has been kept in the net of a pearl farm, it did not have the freedom to do so. Photo © H.A. Hänni, SSEF and GemExpert.
Figure 5. A dead symbiotic oyster crab (left: down side, right: upside) was buried under the mantle and then subsequently covered with nacre. In nature, the shell would have expelled the crab by just flushing it out. As the P. maxima oyster has been kept in the net of a pearl farm, it did not have the freedom to do so. Photo © H.A. Hänni, SSEF and GemExpert.

Wild oysters can often travel quickly by clapping the two wings. With such motions they would definitely get rid of disturbing objects under the mantle. Farmed oysters kept in nets would not have that liberty; therefore the fish blister can hardly be of natural formation. In ‘The book of the pearl’ by Kunz and Stevenson (1908) . That the encysted fish in the same book (opposite p. 42) is a natural formation can be questioned, as the technique of sliding objects for a nacre coating under the mantle has been known long before. The classic mabé pearls (cultured pearl doublet) is another example of the handling that the oyster exercises with objects placed between mantle and shell. The hemisphere glued under the mantle of older P. maxima is readily coated. Later the lump is cut out and closed with a nacre base on its rear side. (source: Keshi pearls wholesale)

On the other hand, there are natural blisters that represent the mantle’s effort to keep an intruder away from the soft body. When an attack of a drilling worm is sensed, an increased amount of nacre can be secreted on the spot where the penetration is expected. A thick lump in the shell is the result of such defence. Even when the worm has reached the inner part of the shell, the fight may go on. A typical reaction of the mantle would be the formation of a conchiolin coat on the intruder, followed by calcium carbonate in primitive form as spherulites (prismatic CaCO3 in radiating array) composed by columnar aragonite. Figures 6a and 6b show an example of such an incident. (source: Keshi pearls wholesale)

Figure 6a. A cut open blister on a wild Pinctada radiata shell (Bahrain). The horizontal layer is the shell, drill hole and accumulation is visible in the centre. In the hollow space was organic material. Width 7 mm. Photo © H.A. Hänni, SSEF and GemExpert.
Figure 6a. A cut open blister on a wild Pinctada radiata shell (Bahrain). The horizontal layer is the shell, drill hole and accumulation is visible in the centre. In the hollow space was organic material. Width 7 mm. Photo © H.A. Hänni, SSEF and GemExpert.
Figure 6b. SEM picture of details from the surface on the inside of the blister in Figure 6a. On the back is conchiolin with drying fissures (lower left). Elongated aragonite sticks forming aggregates up to spherulites. Magnification 400x. Photo © Marcel Düggelin, ZMB, Basel.
Figure 6b. SEM picture of details from the surface on the inside of the blister in Figure 6a. On the back is conchiolin with drying fissures (lower left). Elongated aragonite sticks forming aggregates up to spherulites. Magnification 400x. Photo © Marcel Düggelin, ZMB, Basel.

Mantle-grown natural pearls All natural pearls are understood as formations subsequent to mantle injuries that lead to the formation of a pearl sac by displaced external mantle cells (Figure 7a). By the injury through an animal attack some of the external mantle cells are moved into the conjunctive tissue, the layer somewhat deeper in the mantle. Here the cells may stay alive and constitute a cyst or pearl sac. All external mantle cells are born to secrete calcium carbonate. On the pocket’s inner side the precipitation of calcium carbonate thus starts, forming a small accumulation that may grow to a pearl. (source: Keshi pearls wholesale)

Figure 7a. Schematic diagram showing the shell and the mantle related. The pink layer represents the external mantle tissue whose cells have the capacity of forming CaCO3. The mantle tissue is always sitting on its production: on the right side the old tissue has produced the thickest wall. © H.A. Hänni, SSEF and GemExpert.
Figure 7a. Schematic diagram showing the shell and the mantle related. The pink layer represents the external mantle tissue whose cells have the capacity of forming CaCO3. The mantle tissue is always sitting on its production: on the right side the old tissue has produced the thickest wall. © H.A. Hänni, SSEF and GemExpert.

The pocket grows as the pearl is increasing in size, and is now called pearl sac (Figure 7b). All natural pearls are mantle-grown, as the mantle is the only organ that is able to secrete CaCO3 and thus form pearls without human intervention. This explanation of natural pearl formation excludes the wide spread sand-grain theory mainly because sand grains are inactive and never actively intrude into the outer mantle tissue (Hänni, 2002). (source: Keshi pearls wholesale)

Figure 7b. Schematic diagram explaining natural pearl formation. The white layer is the conjunctive tissue that can accommodate accidentally displaced cells. They may form a pearl sac that contains the CaCO3 precipitation. The injury usually affects young mantle tissue at the rim of the shell. Cells of that juvenile age are secreting columnar CaCO3, as found in many natural pearls in the core. © H.A. Hänni, SSEF and GemExpert
Figure 7b. Schematic diagram explaining natural pearl formation. The white layer is the conjunctive tissue that can accommodate accidentally displaced cells. They may form a pearl sac that contains the CaCO3 precipitation. The injury usually affects young mantle tissue at the rim of the shell. Cells of that juvenile age are secreting columnar CaCO3, as found in many natural pearls in the core. © H.A. Hänni, SSEF and GemExpert

Mantle-grown cultured pearls Mantle-grown cultured pearls originate from a transplant of external mantle tissue into the conjunctive tissue of a recipient shell (Hänni, 2007). This small tissue piece, when grafted, folds back and transforms into a pearl sac. Evidences for this process are all those mantle-grown cultured pearls without a bead, typically formed in Chinese freshwater mussels. As a rule several pieces of tissue are arranged in three rows in each wing of the mussel. Herewith, up to 50 pearls can be harvested after a period of time (Figure 8). (source: Keshi pearls wholesale)

Figure 8. A just opened freshwater mussel from Donggou (China) showing a large number of beadless cultured pearls in the mantle of one wing. Length of the shell is approx. 16 cm. Photo © H.A. Hänni, SSEF and GemExpert.
Figure 8. A just opened freshwater mussel from Donggou (China) showing a large number of beadless cultured pearls in the mantle of one wing. Length of the shell is approx. 16 cm. Photo © H.A. Hänni, SSEF and GemExpert.

A further development of mantle-grown cultured pearls is the use of a flat coin-shaped nucleus, seen in round, square, etc. shapes. The grafting was performed at the outer part of the shell, easy to reach from outside, where the two shell halves are still close. That the same pearl sac is used later for housing a
spherical bead is just a clever advancement. The further growth of the shell after the flat bead introduction has moved the pearl sac deeper into the shell where a spherical bead can now find enough space (Fiske and Shepherd, 2007).  Results of this mantle process are the classical beadless cultured pearls like Biwa, China freshwater and Mississippi. Similar mantle-grown products from saltwater oysters (except Akoya keshis) are presently only suspected (Hänni, 2008).(source: Keshi pearls wholesale)

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. (source: Keshi pearls wholesale)
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