Pearl cultivation process
Pearl, unlike gemstones or precious metals, is a kind of lively gem. A natural pearl forms when an irritant, such as parasite or a piece of sand, accidentally enters the body of a particular species of oyster, mussel or clam and cannot be expelled. As a defense mechanism, the mollusk secretes a smooth, crystalline fluid, called ¡°nacre¡± (the same secretion it uses for shell-building, composed mainly of carbonated calcium), to coat the intruder. As long as the irritant is present, the mollusk continues to add a layer upon a layer of nacre on the irritant until a lustrous pearl is formed.
A cultured fresh water pearl, which is grown in a clam undergoes exactly the same process. The only differentce is that the irritant is a surgically implanted piece of mantle tissue. To culture fresh water pearls, skilled technicians slightly open a host clam¡¯s shells up to 1 - 1.2 cm, cut small slits into the mantle tissue inside both shells, insert a small piece of epthelial membrane (the lip of mantle tissue) from another clam into those slits, and using a fine needle to shape it into round. After implanting, it takes five to seven days for a host clam to cover an irritant with its own tissue and 10 days later it begins producing centric layers of nacre. Tendering pearl-bearing clams is a continuous process aiming at smoothly developing the pearls to minimize blemishes. The pains-taking process can last four to five years long.
Apparently, in fresh water clams, the insertion alone is sufficient to start nacre production. No artificial nucleating bead is needed. Therefore cultured fresh water pearls are composed entirely of pure nacre, making them ¡°real¡± pearl, just like their natural fresh water and natural salt water counterparts.
Fresh water pearls
Although the traditional source of pearls has been oysters which live in saltwater, mollusks which live in freshwater lakes and rivers can also produce pearls. China has harvested freshwater pearls for many a millennia. The first record mentioning pearls in China was from 2206 BC. The United States was also a major source of freshwater pearls from the discovery of the New World up through the 19th century, when over-harvesting and increasing pollution significantly reduced the number of available pearl-forming mussels.
Freshwater pearls are often somewhat less lustrous than their salt water counterparts. However, they appear in a wide variety of shapes and colors, and they tend to be less expensive than saltwater pearls, making them quite popular. Freshwater pearls are also quite durable, resisting chipping, wear, and degeneration.
Freshwater pearls differ from other cultured pearls in that they are not bead-nucleated. Freshwater mollusks are nucleated by creating a small incision in the fleshy mantle tissue and inserting a piece of mantle tissue from another oyster. This process may be completed 25 times on either side of the mantle, producing up to 50 pearls at a time. The mollusks are then returned to their freshwater environment where they are tended for 2-6 years. The resulting pearls are of solid nacre, but without a bead nucleus to guide the growth process, the pearls are rarely round.
In recent years the Chinese have been able to take the art of culturing freshwater pearls to new levels. In the last decade the quality of pearls produced have become so high that many pearls in the top percentage of a harvest are nearly indistinguishable of their saltwater relatives. Gone are the rice-shape seed pearls as they are now being replaced with round, lustrous pearls of sizes as large as 16mm, mimicking large South Sea pearls. This has created a renewed interest in freshwater pearls as an affordable alternative to the higher priced saltwater.
The Japanese have a distinguished history of culturing freshwater pearls as well. Lake Biwa was once world renowned for producing high-quality freshwater pearls. However, in the mid 1970's pearl farming all but came to a halt due to pollution in this lake that was once synonymous with freshwater pearls. Today the Japanese are trying once again to farm freshwater pearls in Lake Kasumigaura, utilizing a bead-nucleated hybrid mussel ( Hyriopsis Schlegeli anadonata/plicata hybrid mussels). The resulting pearls have been quite large and unique. But due to the high prices of such pearls the market remains a niche for collectors.
Fresh water vs. Akoya
Freshwater and Akoya pearls, while both genuine pearls, differ greatly in value and composition. The three main differences are the culturing process, the nucleus, and the shape.
Fresh water pearls are mantle-tissue nucleated, whereas Akoya pearls are bead-nucleated. Instead of inserting a mother of pearl bead and a piece of mantle tissue into the gonad of a freshwater mollusk as is the process with an Akoya oyster, only a piece of mantle-tissue is used, and this is inserted into the mantle tissue of the freshwater mollusk, not the gonad. The result is a pearl composed of solid nacre, and the mantle tissue is eventually dissolved or drilled out.
Fresh water pearls are nucleated in the mantle tissue which is on either side of the oyster. This tissue is much larger than the gonad of an Akoya oyster. Therefore the freshwater mollusk can be nucleated up to 25 times on either side, for a total of 50 nucleations. An Akoya oyster, on the other hand, can handle a maximum of 5 nucleations in its gonad, but very rarely is nucleated with more than 2 beads at a time. So upon harvest, a freshwater mollusk may produce up to 50 pearls at a time, while the Akoya oyster has a maximum production of 2.
Freshwater mollusks are also much easier to farm. The mortality rate is much lower than that of the nucleated Akoya oysters, and fresh water farms rarely deal with natural disasters such as typhoons and red tides that plague Akoya pearl farms. (see pearl guide)
South Sea pearls
South Sea pearls are among the largest commercially harvested cultured pearls in the world. The average size of a South Sea pearl is 13mm, with most harvests producing a range of sizes from 9mm up to 20mm.
The South Seas lie between the northern coast of Australia and the southern coast of China. These waters are the native habitat of the large oyster, the Pinctada maxima. This oyster grows up to 12 inches in diameter, and can be nucleated with a much larger bead than other saltwater oysters such as the Akoya.
There are two varieties of Pinctada maxima: the silver-lipped and the gold-lipped. The two are distinguished by the coloration of the outer edge of the interior. This shell is also known as mother of pearl, and is responsible for the coloration of the cultured pearls produced.
Unlike the Akoya oyster, the South Sea oyster will only accept one nucleation at a time. The oyster is nucleated when it is only about half developed, from 4.7 inches to 6.7 inches in size, or about 24 months old. Although the South Sea oyster will only handle one nucleus at a time, this oyster (like the Tahitian pearl producing Pinctada margaritifera) can be nucleated up to three times over the course of many years.
There are four reasons South Sea pearls can grow to such large sizes dwarfing many of their other salt water pearl counterparts. The reasons consist of: the large size of the Pinctada maxima, the size of the implanted bead, the length of time the pearl is left to grow in the oyster, and the oyster¡¯s environment. Due to the size of the oyster it is able to accept a large bead. The gonad of the Pinctada maxima is several times larger than that of the Akoya. Because of this same reason the South Sea oyster deposits nacre around the nucleus at a much quicker rate, especially in warm water which speeds the oyster¡¯s metabolism. The South Seas are also extremely clean, and filled with plankton - the Pinctada maxima¡¯s favorite food source. The clean waters and abundant food supply also speeds the nacre production. The growth period for South Sea pearls is also substantially longer than that of the Akoya. Akoya pearls are harvested after only 9-16 months, where as South Sea pearls are harvested after at least 2 years.
South Sea pearls have several distinct characteristics that are unique to this gem. The nacre is unusually thick, ranging from 2-6mm compared to the .35-.7mm of an Akoya pearl. South Sea pearls also have a unique, satiny lustre that comes from the rapidly deposited nacre and warm waters of the South Seas. South Sea pearls also have a subtle array of colors, typically white, silver, and golden, that are rare in other pearl types. (see pearl guide)