   #copyright

Pearl

2007 Schools Wikipedia Selection. Related subjects: Everyday life

   White pearls strung on a necklace
   Enlarge
   White pearls strung on a necklace

   A pearl is a hard, rounded object produced by certain animals,
   primarily mollusks such as oysters. Pearls can be used in jewelry and
   also crushed in cosmetics or paint. Pearl is valued as a gemstone and
   is cultivated or harvested for jewelry.

   Pearls are formed inside the shell of certain bivalve mollusks. As a
   response to an irritant inside its shell, the mollusk will deposit
   layers of calcium carbonate (CaCO[3]) in the form of the minerals
   aragonite or calcite (both crystalline forms of calcium carbonate) held
   together by an organic horn-like compound called conchiolin. This
   combination of calcium carbonate and conchiolin is called nacre, or as
   most know it, mother-of-pearl. The commonly held belief that a grain of
   sand acts as the irritant is in fact rarely the case. Typical stimuli
   include organic material, parasites, or even damage that displaces
   mantle tissue to another part of the animal's body. These small
   particles or organisms enter the animal when the shell valves are open
   for feeding or respiration. In cultured pearls, the irritant is
   typically a cut piece of the mantle epithelium, together with processed
   shell beads, the combination of which the animal accepts into its body.

   The unique lustre of pearls depends upon the reflection and refraction
   of light from the translucent layers and is finer in proportion as the
   layers become thinner and more numerous. The iridescence that some
   pearls display is caused by the overlapping of successive layers, which
   breaks up light falling on the surface. Pearls are usually white,
   sometimes with a creamy or pinkish tinge, but may be tinted with
   yellow, green, blue, brown, purple, or black. Black pearls, frequently
   referred to as Black Tahitian Pearls, are highly valued because of
   their rarity; the culturing process for them dictates a smaller volume
   output and can never be mass produced. This is due to bad health and/or
   non-survival of the process, rejection of the nucleus (the small object
   such as a tiny fish, grain of sand or crab that slips naturally inside
   an oyster's shell or inserted by a human), and their sensitivity to
   changing climatic and ocean conditions.

   The largest pearl ever found, so far, came from the Philippines in
   1934. It weighed 14 lbs (6.36 kgs) when it was discovered by an
   anonymous Muslim Filipino diver off the island of Palawan. Later, a
   Palawan chieftain gave the pearl to Wilbur Dowell Cobb in 1936 as gift
   for having saved the life of his son. It was first called the Pearl of
   Allah and is now officially named the Pearl of Lao-tze.

   Pearls fit into two categories: freshwater and saltwater. As their name
   implies, freshwater pearls are formed in freshwater mussels that live
   in lakes, rivers, ponds and other bodies of fresh water. Most
   freshwater cultured pearls sold today come from China. By contrast,
   saltwater pearls grow in oysters that live in the ocean, usually in
   protected lagoons. Akoya, South Sea and Tahitian are the three main
   types of saltwater pearls.

History

   Pearl farm, Seram, Indonesia
   Enlarge
   Pearl farm, Seram, Indonesia

   Before the beginning of the 20th Century, pearl hunting was the most
   common way of harvesting pearls. Divers manually pulled oysters from
   ocean floors and river bottoms and checked them individually for
   pearls. Not all natural oysters produce pearls, however. In fact, in a
   haul of three tonnes, only three or four oysters will produce perfect
   pearls.

   Now, however, almost all pearls used for jewelry are cultured by
   planting a core or nucleus into pearl oysters. The pearls are usually
   harvested three years after the planting, but it can take up to as long
   as six years before a pearl is produced. This mariculture process was
   first developed by Kokichi Mikimoto in Japan, who was granted a patent
   for the process in 1896. The nucleus is generally a polished bead made
   from mussel shell. Along with a small scrap of mantle tissue from
   another oyster to serve as an irritant, it is surgically implanted near
   the oyster's genitals. Oysters which survive the subsequent surgery to
   remove the finished pearl are often implanted with a new, larger
   nucleus as part of the same procedure and then returned to the water
   for another three years of growth.

   The original Japanese cultured pearls, known as Akoya pearls, are
   produced by a species of small oysters no bigger than 6 to 7 cm in
   size, hence Japanese pearls larger than 10 mm in diameter are extremely
   rare and highly prized. In the past couple of decades, cultured pearls
   have been produced with larger oysters in the south Pacific and Indian
   Ocean. One of the largest pearl-bearing oysters is the Pinctada maxima,
   which is roughly the size of a dinner plate. South Sea pearls are
   characterized by their large size and silvery colour. Sizes up to 14 mm
   in diameter are not uncommon. Australia is one of the most important
   sources of South Sea pearls. Tahitian pearls (also referred to as
   Titian pearls) are also another South Sea pearl.

   In 1914 pearl farmers began culturing freshwater pearls using the pearl
   mussels native to Lake Biwa. This lake, the largest and most ancient in
   Japan, lies near the city of Kyoto. The extensive and successful use of
   the Biwa Pearl Mussel is reflected in the name Biwa pearls, a phrase
   nearly synonymous with freshwater pearls in general. Since the time of
   peak production in 1971, when Biwa pearl farmers produced six tons of
   cultured pearls, pollution and overharvesting have caused the virtual
   extinction of this animal. Japanese pearl farmers now culture a hybrid
   pearl mussel—a cross between the last remaining Biwa Pearl Mussels and
   a closely related species from China—in other Japanese lakes.
   Nuclei from Toba Pearl Island, Japan
   Enlarge
   Nuclei from Toba Pearl Island, Japan

   In the 1990s, Japanese pearl producers also invested in producing
   cultured pearls with freshwater mussels in the region of Shanghai,
   China, and in Fiji. Freshwater pearls are characterized by the
   reflection of rainbow colors in the luster. Cultured pearls are also
   produced using abalone.

Jewelry

   Girl with a Pearl Earring, by Jan Vermeer van Delft
   Enlarge
   Girl with a Pearl Earring, by Jan Vermeer van Delft

   The value of the pearls in jewelry is determined by a combination of
   the luster, colour, size, lack of surface flaw and symmetry that are
   appropriate for the type of pearl under consideration. Among those
   attributes, luster is the most important differentiator of pearl
   quality according to jewelers. All factors being equal, however, the
   larger the pearl the more valuable it is. Large, perfectly round pearls
   are rare and highly valued. Teardrop-shaped pearls are often used in
   pendants. Irregular shaped pearls are often used in necklaces.

   Pearls come in eight basic shapes: round, semi-round, button, drop,
   pear, oval, baroque, and ringed. Perfectly round pearls are the rarest
   and most expensive, and are generally used in necklaces, or strings of
   pearls. Semi-rounds are also used in necklaces or in pieces where the
   shape of the pearl can be disguised to look like it is a perfectly
   round pearl. Button pearls are like a slightly flattened round pearl
   and can also make a necklace, but are more often used in single
   pendants or earrings where the back half of the pearl is covered,
   making it look like a larger, round pearl.
   Woman with a pearl necklace, by Jan Vermeer van Delft
   Enlarge
   Woman with a pearl necklace, by Jan Vermeer van Delft

   Drop and pear shaped pearls are sometimes referred to as teardrop
   pearls and are most often seen in earrings, pendants, or as a centre
   pearl in a necklace. Baroque pearls have a different appeal to them
   than more standard shapes because they are often highly irregular and
   make unique and interesting shapes. They are also commonly seen in
   necklaces. Ringed pearls are characterized by concentric ridges, or
   rings, around the body of the pearl.

   In general, cultivated pearls are less valuable than natural pearls,
   and imitation pearls are the least expensive. One way that jewellers
   can determine whether a pearl is cultivated or natural is by x-raying
   the pearl. If the grit in the centre of the pearl is a perfect sphere,
   then the jeweller knows it is cultivated. This is because when the
   cultivators insert the grit, (usually a polished piece of mussel
   shell), it is always perfectly round, so as to produce a more
   expensive, perfectly round pearl. If the centre is not perfectly round,
   the jeweller recognises that it is genuine, and gives it a higher
   value. Imitation pearls are much easier to identify by jewellers. Some
   imitation pearls are simply made of mother-of-pearl, coral or conch,
   while others are made from glass and are coated with a solution
   containing fish scales called essence d'Orient. Although imitation
   pearls look the part, they do not have the same weight or smoothness as
   real pearls, and their luster will also dim greatly.
   Pearl Market in Beijing, China
   Enlarge
   Pearl Market in Beijing, China

   There is also a unique way of naming pearl necklaces. While most other
   necklaces are simply referred to by their physical measurement, strings
   of pearls have their own set of names that characterize the pearls
   based on where they hang when worn around the neck. A collar will sit
   directly against the throat and not hang down the neck at all, they are
   often made up of multiple strands of pearls. Pearl chokers nestle just
   at the base of the neck. The size called a princess comes down to or
   just below the collarbone. A matinee of pearls falls just above the
   breasts. An opera will be long enough to reach the breastbone or
   sternum of the wearer, and longer still, a pearl rope is any length
   that falls down farther than an opera.

   Necklaces can also be classified as uniform, where all the pearls are
   the same size, graduated, where the pearls are arranged in size from
   large in the centre to smaller at the ends, or tin cup, where pearls
   are generally the same size, but separated by lengths of chain.
   Retrieved from " http://en.wikipedia.org/wiki/Pearl"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
