Pearls are hard objects grown within the soft tissue of bivalve mollusks. A pearl is formed when a microscopic foreign object (usually a parasite) becomes lodged in the soft mantle tissue of the host animal. In response, the mollusk secretes nacre, made from the same calcium carbonate that makes up the ordinary shell of a mollusk, in concentric layers around the irritation, forming a pearl.
Although almost all of the 8000 different species of bivalve mollusks can produce some kind of pearl, only two groups generate gem quality material. Pearls from the sea come from a range of species collectively known as pearl oysters (Family Pteriidae), while the less valuable freshwater pearls are obtained from a few species of the freshwater pearl mussel (Order Unionida).
A natural pearl by definition is one that forms in the wild without any human intervention and appears only very rarely, with approximately one occurrence per ten thousand oysters. Because the layers of nacre tend to maintain their irregular shape around the original irritant, natural pearls that are spherical in shape are even more rare. As a result, pearls have been highly valued as gemstones for millennia and the very word "pearl" has become synonymous with any object considered rare, fine and valuable.
During the early part of the 20th century, a technique was developed for artificially culturing pearls which revolutionised the pearling industry. A foreign substance called a nucleus is inserted into the soft tissue of the bivalve, which is then returned to the water to grow. This technique can be applied to both freshwater and marine varieties and is now the primary means of obtaining pearls all over the world. Since the advent and development of the cultured pearl industry, pearls have become much more affordable. However, a set of matched natural peals can still command hundreds of thousands of dollars.
The perfect pearl is perfectly round and smooth, but many other shapes of pearls (called baroque) occur, some of which are today considered more desirable as fashions and perceptions of fashion change. Natural or cultured, pearls from the sea are traditionally valued more highly than freshwater pearls. The value of the pearls in jewellery is determined by a combination of luster, colour, size, lack of surface flaw and symmetry. All factors being equal, however, the larger and rounder the pearl the more valuable it is. Luster is also an important differentiator of pearl value: the more numerous the layers of nacre, the finer the lustre. Pearls can also occur naturally in many colours. Black pearls in particular are highly valued because of their rarity; the specific culturing process for them means they can never be mass-produced. However today pearls, especially the cultured freshwater variety, can be dyed any colour desired.
The fabulous Canning Jewel. Crafted from solid gold, diamonds, rubies and pearls and thought to have been made during the Renaissance, it was gifted to Lord Canning in India for his services to the country. The Victoria & Albert Museum in London purchased the jewel in 1935 for the then princely sum of $26,000
Cultured pearls can only be distinguished from natural pearls by x-ray examination, although it is safe to say that all pearls currently for sale on the market are cultured. Imitation or fake pearls are also widely sold today for use in inexpensive jewellery, but the quality of their lustre is generally poor and they may easily be distinguished from genuine pearls, cultured or natural. Imitation pearls are generally made of mother-of-pearl, coral or conch shell, while others are made from glass and are coated with a solution containing fish scales called essence d'Orient. They lack the weight of real pearls, and they also deteriorate in appearance with time. An easy way to distinguish a real pearl from a fake is to rub it gently backwards and forwards against the teeth. If the result is smooth, the pearl is artificial. If the pearl feels rough and gritty, the pearl is genuine.
Mary Queen of Scotts'famous rope of black pearls,stolen from her
when she fled to England
Historically pearls have been used not only for jewellery making but were also used to enhance buildings, clothing, headwear and other personal items, and were also crushed for use in cosmetics, medicines and paint formulations. The pearl is perhaps unique in being named in the sacred texts of all the worlds major religions: the Hindu, Hebrew, Islamic and Christian scriptures all make mention of pearls in metaphor, parable and as a promise of just rewards to the devout. Many famous pearls exist in history. Mary Queen of Scotts owned a famous rope string of black pearls, stolen from her when she fled to England and subsequently purchased by Elizabeth I from the courtier responsible for the theft. One of the largest ever known pearls was famously dissolved in wine and drunk by Cleopatra to impress Mark Anthony, although this was a common practice at the time amongst wealthy Romans wishing to entertain their friends. The Canning jewel is a famous piece of jewellery from the 16th century containing a large baroque pearl used as the sea god Triton's torso. The largest pearl ever found is the Pearl of Allah, taken from a giant clam in 1934 off the coast of the Philippines. Weighing almost 7 kg, the pearl is valued at US$40 million. The Hope Pearl, named for it's owner Henry Philip Hope (also the owner of the Hope Diamond) is thought to be thelargest natural saltwater pearl ever discovered. Weighing 450 carats, the Hope is a white drop-shaped blister pearl, measuring approximately 2 x 4 inches. It currently resides in the British Museum of Natural History.
Officially the largest pearl in the world, the Pearl of Allah is porcelain-like
without iridescence or nacre
As organic gems, pearls should be treated with particular care. They should never be washed or immersed in water, left in sunlight, exposed to strong perfumes or exposed to household chemicals - all of these can wear away the nacre and cause color change. As a rule of thumb, pearls should be restrung once every couple of years or as soon as the stringing thread shows signs of fraying or a build-up of grime. Wipe your string of pearls with a soft dry cloth after every outing and then store safely away until next time. Because pearls are very soft they are best stored in soft cotton or chamois pouches - never in plastic bags. Each piece of pearl jewellery that you own should be stored separately and away from other kinds of jewellery that may scratch and abrade your pearls even within their pouches.
References and Further Reading
Pearl. Wikipendia. Accessed on 10th June 2010
PearlGuide.com. Accessed on 11th June 2010
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