Pearl: history and origin
1.3.1 Natural pearls
Natural pearls were first discovered accidentally when human searched for food along the coastline and in lakes and rivers in prehistoric time (Dakin, 1913; Kunz & Stevenson, 1908; Strack, 2006; Ward, 1995). Pearls subsequently became parts of the rituals associated with cultural and religious activities (Strack, 2006).
The shells of pearl bearing species or mother of pearl (MOP) has also been utilised for decoration throughout human history. MOP inlays were used around 4500 BC in Mesopotamia and around 4000 BC in Egypt (Strack, 2006; Ward, 1995). The use of pearls for decoration was assumed to have begun in the 5th Century BC during the Persian invasion (Kunz & Stevenson, 1908) where possibly the oldest necklace containing pearls was found in a sarcophagus at the Winter Palace of the Persian kings in Susa (Strack, 2006).
During the Roman Era pearls became the most valuable gems (Ward, 1995). The ‘Pearl Age’ began in the 16th century. Although there was a shift from pearls to diamonds in the 18th century, pearls regained their top position in the nineteenth century when new pearl oyster beds were discovered and cultured pearl production began (Strack, 2006).
Many believe that natural pearls are formed as a reaction to an irritant in the internal part of a mollusc (Kunz & Stevenson, 1908; Strack, 2006; Streeter, 1886; Ward, 1995). The irritant may be a trapped parasites, small particles, or mantle scratches due to friction or predator damage (Strack, 2006; Ward, 1995). However, pearls will not be formed without the existence of epithelial cells from the nacre secreting mantle tissue (Simkiss & Wada, 1980). Therefore, for a pearl to form the irritant (other than mantle epithelium) must be associated with some epithelial mantle (Strack, 2006). The epithelial cells begin to proliferate and form a ‘pearl-sac’ to cover the irritant (Taylor & Strack, 2008). The pearl-sac then begins to deposit minerals (nacre) as a kind of internal defence mechanism (Dakin, 1913; Kunz & Stevenson, 1908). Such deposition continues and the resulting pearl grows. The shape of the irritant is usually irregular and this irregularity causes pearls to grow asymmetrically in shape (baroque type). This type of pearl is common in natural pearls (Strack, 2006).
Other types of natural pearls may also be formed on the internal surface of the shell. They are called blisters (Taylor & Strack, 2008). The formation of natural blisters results from the reaction of the host to organisms that penetrate the shells or any material trapped between mantle and the shell (Kunz & Stevenson, 1908). The penetration is mainly caused by boring sponges, Cliona spp. (Fromont et al., 2005), boring polychaetes, Polydora spp. (Alagarswami and Chellam, 1976; Okoshi and Sato-Okoshi, 1996) and several lithopagan bivalves (Doroudi, 1994; Takemura and Okutani, 1956). Borers are usually categorised as pests in the cultured pearl industry because they may kill the oysters (Che et al., 1996; Humphrey, 2008; Humphrey & Norton, 2005; Jones, 2007). In response to shell penetration, the host begins to secrete nacreous material to cover the resulting damage or irritation on the inside of the shell. This process results eventually in the production of a blister (Taylor & Strack, 2008).
Natural pearls are very rare and occur in approximately one in a thousands oysters (Haws, 2002). However, the frequency with which natural pearl occurs varies according to species and the region in which they are found. In the nineteenth century one high-valued pearl could be found at a ratio of 500:1 in the Persian Gulf, 5000:1 in the Sulu Sea, 15.000:1 in French Polynesia, and 1.000.000:1 in the Gulf of Manaar, Ceylon (Strack, 2006). Obtaining such pearls is usually costly. Pearl divers are susceptible to accidents and shark attacks (Joyce & Addison, 1992; Kunz & Stevenson, 1908). Such conditions made pearls in 19th and 20th century among the most expensive gems which were restricted to the rich and to noblemen in particular (Dakin, 1913; Ward, 1995).
Before the early 20th Century, there were several places with large beds of oysters and mussels that supported a pearl fishery. In the marine environment, large pearl oyster beds stretched from Arabian waters to the Pacific and there were smaller, patchy distributions in Central America (Kunz & Stevenson, 1908). Pinctada radiata1 (synonym P. imbricata1) were abundant in the Persian Gulf and the Gulf of Manaar (Sri Lanka), P. fucata1(synonym P. martensii1) in Japanese waters, P. mazatlanica in Pacific Central America, P. margaritifera in the south Pacific and P. maxima in the tropical central Indo-Pacific region (Strack, 2006). Particular regions like the Persian Gulf, Gulf of Manaar and northern Australia were famous for their natural pearl fisheries.
In freshwater, smaller scale pearl fisheries were mainly distributed in the northern hemisphere with Margaritifera margaritifera being the main species. However, nowadays freshwater pearl mussel beds in Europe have been depleted and are being conserved (Bauer,1 There is some confusion over the taxonomic status of these taxa (Wada and Tëmkin, 2008). 1988; Beasley & Roberts, 1996; Cosgrove et al., 2000; Cosgrove & Hastie, 2001; Young, 1991).
1.3.2 Cultured pearls
Valuable pearls are usually produced from molluscs that have a nacreous lining (MOP) to the inner surface of their shells (Webster, 1994). These molluscs are selected for pearl production. For cultured marine pearls, the commonly cultured species are from the family Pteriidae: Pinctada maxima, P. margaritifera, P. fucata and Pteria penguin (Gervis & Sims, 1992; Southgate et al., 2008b). Marine pearls are cultivated mainly in the Indo-Pacific region; from the Red Sea to the Pacific Ocean (Bondad-Reantaso et al., 2007; Strack, 2006). Japan is famed for Akoya cultured pearls produced by Pinctada fucata; Indonesia and Australia lead ‘South Sea’ pearl production from P. maxima and the Pacific island countries produce ‘Tahitian’ cultured pearls from P. margaritifera (Southgate, 2007).
The first innovation towards cultured pearl production was introduced by the Chinese in the 5th Century who produced blister pearls in the shape of Buddha (Joyce & Addison, 1992). This was carried out using freshwater mussels. More than a thousand years later in Europe, Linneaus conducted experiments by creating a hole in the shells of the river mussels, Unio pictorum, into which he put a limestone nucleus attached to a wire in the shell (Strack, 2006). He then left the mussels in the water for five years, however, the resulting pearls were of very poor quality (Joyce & Addison, 1992). Several attempts to produce pearls were conducted by William Saville-Kent on Pinctada maxima in 1890, followed by Kokichi Mikimoto three years later on hemispherical pearls (George, 1996; Simkiss & Wada, 1980). In 1914, Mikimoto applied for a patent for producing round cultured pearls (George, 1967) and he received it two years later (Ward, 1995). This modern method of culturing round pearl production utilised a nucleus wrapped within a piece of mantle tissue, which was implanted into the gonad of a recipient oyster. This method was actually invented by Saville-Kent and adopted by Tatsuhei Mise and Tokichi Nishikawa, but Mikimoto claimed the patent (George, 1996; Matlins, 2002). The method was subsequently applied commercially in 1919 (Simkiss & Wada, 1980; Taylor & Strack, 2008).
Article source: Mamangkey, Noldy (2009) Improving the quality of pearls from Pinctada maxima. PhD thesis, James Cook University.
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