BY ARTHUR THOMAS
The freshwater pearls that are recovered from various varieties of unio mussels have enjoyed a long history of use in items of personal adornment.
The Romans, under Julius Caesar, invaded the Cassiterides or tin islands (Britain) in 55 BC in search of plunder and slaves. In addition to tin and zinc the islands were an important source of gold, silver, lead, wool and corn.
However the Romans did not discover the river pearls until 43 AD when the Emperor Claudius decided to occupy the islands and incorporate them into the Roman Empire.
Left to right: Irradiated Fresh-water, Biwa dragon pearls (monsters)
The islands were sparsely populated and the rivers flowed clean and sweet with little or no pollution. The quality of the river pearls that were collected and sent to Rome was so high that they were soon in demand.
Today the only ongoing British pearl production comes from the River Dee in Scotland. Fishing there is very restricted and the queen has the right of first refusal on any pearls that are found.
Lake Biwa in Japan was another source of very fine Unio pearls that were found in a wide range of beautiful pastel hues. Since the production recovered from “wild” mussels was very limited an industry sprang up that involved farming the mussels. The mussels were much larger and longer lived than the marine “oysters” (actually Akoya pinctada or wing-shells) that are farmed in Japan, so it was possible to grow numerous pearls in a single organism. When a captive mussel was operated on numerous small fragments of outer mantle tissue were inserted into its body. The process of culturing non-nucleated cultured pearls was initially developed at Biwa. Mussels are filtering organisms and very vulnerable to even relatively low levels of pollution. Lake Biwa is situated on Honshu, the principal island of Japan and the pearl farms eventually fell victim to the advance of industrialisation.
Left to right: Biwa pearls belted, Chinese dyed rice-crispie pearls, Fresh-water button pearls, China
For centuries the Buddhist monks of China created small medallions of Lord Buddha that were cast in lead and then coated with pearly nacre. Slivers of bamboo were used to make an opening between the body of a mussel and its shell so they could insert a small lead medallion before returning it to the stream-bed. The mussels were left for a protracted period before the monks lifted them to remove the coated medallions.
The initial production of tissue-nucleated fresh-water culture from China bore a marked resemblance to rice-crispies. They were obviously very inexpensive being dyed on the strand in unrealistic hues. Pearls do not accept dye uniformly therefore a better product could have been created by selecting and matching the pearls before they were made up into strands.
Chinese pearl production has improved dramatically since those early days. The shapes are attractive, the coating is uniform and artificial treatment to enhance their colouring is subtle.
The successful culturing of fine quality spherical mussel pearls was a major breakthrough for the Chinese industry. The process involved requires both skill and a great deal of patience.
Fortunately the Chinese are famous for their possession of the later virtue and unio mussels are a very long-lived species. In fact scientific estimates of their life span range from seventy to one hundred and thirty years.
To briefly summarise the process that is involved in producing the round pearls. Coin-shaped nuclei accompanied by sections of donor mantle tissue are implanted into the mussels. The mussels are then returned to the river or lake where they can recover from the operation and begin building a nacreous coating on the implants. After two years they are lifted, the ‘coin-pearls’ are harvested and spherical nuclei are implanted in the pearl sacs that have been created.
It generally requires a further two years for the spherical nuclei to receive a substantial coating of nacre. This process is capable of yielding fine quality, round to near-round cultured pearls of 10.0 – 15.5mm diameter.
This process is believed to have the capability to produce 15.0 – 20.0mm round to slightly off-round cultured pearls that will make inroads into the South-Sea Pearl Market.
Fellow, Gem-A(GtBr) / Graduate gemologist GIA (USA) / Certified Evaluator (SA)
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