A pearl is a hard, rounded object produced by certain mollusks, primarily oysters. Pearl is valued as a gemstone and is cultivated or harvested for jewellery.
Pearls are formed inside the shell of certain bivalve mollusks. As a response to an irritating object inside its shell, the mollusk will deposit layers of calcium carbonate (CaCO3) 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 unique luster 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 refered to as Black Tahitian Pearls are highly valued because of their rarity; the culturing process for them dictates a smaller volume output due to rejection by the oysters.
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 priced. 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 color. 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.
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.
Development of a Pearl
A pearl is formed when some sort of small object or irritant becomes embedded in the tissue of an oyster or mollusk. In response, the mantle tissue of the mollusk secretes nacre, a combination of crystalline and organic substances. As the nacre builds up in layers, it surrounds the irritant and eventually forms a pearl.
Natural pearls are those pearls which are formed in nature, more or less by chance. Cultured pearls, by contrast, are those in which humans take a helping hand. By actually inserting a foreign object into the tissue of an oyster or mollusk, pearl farmers can induce the creation of a pearl. The same natural process of pearl creation takes place.
The Pearl Industry
Modern-day cultured pearls are primarily the result of discoveries made in the late 19th and early 20th centuries by Japanese researchers, most notably Kokichi Mikimoto. Although some cultures had long been able to artificially stimulate freshwater mollusks into producing a type of pearl, the pearls produced in this way were generally small and misshapen, rather than actual round pearls. What Mikimoto discovered was a specific technique for inducing the creation of a round pearl within the tissue of an oyster.
This discovery revolutionized the pearl industry, because it allowed pearl farmers to reliably cultivate large numbers of high-quality pearls. In contrast to natural pearls -- which have widely varying shapes, sizes, and qualities, and which are difficult to find -- cultured pearls could be "designed" from the start to be round and primarily flawless. The oysters could be monitored for up to two years until each pearl is fully formed, thus better insuring their health and survival. And the pearls could be grown by the tens of thousands, thereby bringing their cost down to a point where pearls became accessible to large numbers of people around the world.
In short, the development of cultured pearls took much of the chance, risk, and guesswork out of the pearl industry, allowing it to become stable and predictable, and fostering its rapid growth over the past 100 years.
Incidentally, prior to the 1930s, exporting pearls was the main economic activity of Kuwait. When the Japanese invented cultured pearls, the Kuwaitis decided to drill for oil. This was the start of the Kuwaiti oil industry.
Cultured pearls can often be distinguished from natural pearls through the use of x-rays, which reveals the inner nucleus of the pearl.
A cultured pearl is a pearl created by a pearl farmer under controlled conditions.
Freshwater pearls are a kind of pearl that comes from freshwater mussels. They are produced in Japan, China, and the United States, and were formerly produced in Scotland.
Freshwater pearls come in various pastel shades of white, black, pink, peach, lavender, plum, purple, and tangerine, depending on the type of mussel.
A single mussel can produce up to 50 pearls. While saltwater pearl-bearing oysters are nucleated in a small organ known as the gonad, freshwater mussels are nucleated in the actual mantle tissue. Each side of this bivalve can handle up to 25 nucleations at one time.
Mother of Pearl
Nacre, also known as mother of pearl or sadaf, is a naturally-occurring organic-inorganic composite. It is formed by layers of calcium carbonate (CaCO3) platelets (crystals), in the form of aragonite and conchiolin (a scleroprotein), separated by elastic biopolymers (such as chitin and lustrin). This mixture of hard and elastic domains makes the material strong and resilient. Strength and resilience are also likely to be due to the offset or "brickwork" arrangement of the platelets, which inhibits transverse crack propagation.
The iridescent appearance of the nacre is due to the fact that the thickness of the aragonite platelets are about 0.5 micrometres, which is comparable to the wavelength of visible light. This results in constructive and destructive interference of different wavelengths of light, resulting in different colors of light being reflected at different viewing angles.
Nacre is secreted by the epithelial cells of the mantle tissue of certain species of mollusk. In these mollusks, nacre is continually deposited onto the inner surface of the animal's shell (the iridescent nacreous layer, commonly known as mother of pearl), both as a means to smooth the shell itself and as a defense against parasitic organisms and damaging detritus.
The iridescent inner layer is considered highly attractive by many cultures and is often used in making jewelery or as inlays in wood furniture and guitars.
When a mollusk is invaded by a parasite or is irritated by a foreign object that the animal cannot eject, a process known as encystation entombs the offending entity in successive, concentric layers of nacre. This process eventually forms what we call pearls and continues for as long as the mollusk lives.
Chief sources are the pearl oyster, found in warm and tropical seas, primarily in Asia; freshwater pearl mussels, which live in many rivers of the United States, Europe, and Asia; and the abalone of California, Japan, and other Pacific regions.
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