Pearl
A natural pearl that can be seen clearly under a gemological microscope is created by bivalve mollusks, primarily oysters, Ostreidae, but also freshwater mussels, and rarely by snails. The pearl is known as La Régente. These filter feeders suck in water and nutrients, along with occasional silt or sand sized grain called an irritant. The irritant may stick to the mantle and the outer skin of the mantle, the epithelium. Although pearl is not an inorganic mineral but rather the organic product of oysters, it is considered by many to be a precious gem. Pearls have been valued for their natural beauty, no faceting or polishing required, for some 6000 years. Pearl quality is determined according to shape, color, size, and luster. The pearl can be clearly seen using a gemological microscope. Spherical are the most valued, but many pearls are irregular shapes, which are termed baroque or barrel pearls. Extreme dryness or moisture are both damaging conditions for pearls, as well as acids, perspiration, cosmetics, and hair spray. The low hardness means pearl is easily scratched and therefore storage is an important consideration.
The layers of nacre, mother-of-pearl, are aragonite, calcium carbonate, and an organic horn type substance, conchiolin, that binds the microcrystalline aragonite around a natural or surgically implanted irritant.
The most popular colors of a pearl are white, pinks, and creams, but pearl may also be silver, golden, red, green, and black. The color varies with the type of mollusk, the water, and the color of the upper conchiolin layer. Treated pearls that can be seen clearly under a gemological microscope are colored peach, lavender, and other colors, while some are bleached to be even whiter. Even though the hardness is only 2.5-4.5, they are compact and difficult to crush.
Pearls vary in size from a pinhead to pigeon’s egg size. The largest ever found, called the Hope Pearl, is 2 inches (5 cm) long, 3.25-4.5 inches (6.5-11.25 cm) in circumference, and weighs 454 carats (about 90 grams). It was named for Henry Philip Hope, one of the owners, who insisted the name be kept as a condition of the sale of the gem (just as he did with the Hope Diamond). It may be seen in the South Kensington Museum in London, the British Museum of Natural History.
Genuine pearls form in nature, whereas cultivated pearls are natural (not imitations!) but produced with human assistance in implanting the irritant. Approximately 90% of the pearl trade today is cultured. Genuine sea pearls are produced by sea mollusks living in warm waters, water depth of about 50-65 feet (15-20 m), and are produced by nonedible varieties of mollusk. Genuine river pearls are not common because pollution has virtually eliminated the oyster habitats. Cultured seawater pearls go back to China, 13th century, and Carl von Linnaeus first produced rounded freshwater pearls about 1721.
The nucleus of the pearl is most often bits of freshwater mussel shell that can be clearly seen using a gemological microscope, from the Mississippi River, fashioned in different shapes and sizes. This shell is wrapped in a piece of tissue from the mantle of a pearl mollusk (Pinetada martensi) and then inserted into the cell lining of the mantle of another pearl mollusk, acting as a pearl sac in which the pearl material is secreted. The most valuable element is the tissue not the irritant, which simply determines the pearl size and shape, and speeds the process. Coreless pearls are on the market, termed Keshi pearls.
The normal bead size is 0.24-0.27 inch or 6-7 mm and when the bead is greater than 0.35 inches or 9 mm, the oyster mortality rate is 80 percent.The mollusk are kept in plastic cages, suspended at 2-6 m (6.5-20 feet) underwater and hung from bamboo floats or ropes fixed to buoys. Yearly pearl growth is dependent upon the water temperature, but may be up to 0.3 to 1.5 mm in warm water and 0.09 mm in cooler water, such as in Japan. The farmed mollusk is in the water 3-4 years for a total layer thickness of 0.8-1.2 mm. Read more
