GO 340 Gemstones & Gemology
 Image taken from CW Jewelers. | Pearls have been valued for their natural beauty, no faceting or polishing required, for some 6000 years. Approximately 70% are strung and worn as necklaces. Pearl quality is determined according to shape, color, size, and luster. 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. |  Cultured Freshwater Pearls. Image taken from Gemstone Originals. |
| A natural pearl is created by bivalve mollusks, primarily oysters, Ostreidae, but also freshwater mussels, and rarely by snails. These filter feeders suck in water and nutrients, along with an 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. The animal then secrets nacre, which is what forms the shell as well as coats the irritant with a nonattached, rounded pearl sac. The layers encrust the irritant, which may lead to a single pearl or a wart-like growth on the inside of the shell, called a blister or shell pearl, under the trade name of mabe pearl. |  Pink and Platinum Cultured Freshwater Pearls. Image taken from Gemstone Originals. |
Purple Pearl. Image taken from the former site, | 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 are white, pinks, and creams, but pearl may also be silver, golden, blue, green, and black. The color varies with the type of mollusk, the water, and the color of the upper conchiolin layer. Treated pearls 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 pin head 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 (Schumann, 1997, p. 230). |  Cultured Pearl Ring. Image taken from the former site, Amazing Pearls. |
 Cultured Pearl Earrings image taken from the former site, Amazing Pearls. | 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 rounded freshwater pearls were first produced by Carl von Linnaeus about 1721. |
| The nucleus of the pearl is most often bits of freshwater mussel shell, 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. |  Tahian Black Pearl. Image taken from the former site, |
 Cultured Pearl Pendant image taken from the former site, Amazing 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 (Schumann, 1997, p. 234). 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. |
| Cultured pearls are treated by bleaching, dyeing, and radiation (not permanent) to alter the color. Pearl imitations are plastic (conspicuously light) to glass (conspicuously heavy). The glass bead is dipped repeatedly in a solution of ground fish scales and the island of Mallorca, Spain, is well known for its imitation pearl industry. The Persian Gulf has been the source of fine natural saltwater pearls, as well as the Micronesian islands, Japan, Sri Lanka, Australia, and Mexico, to name a few. Japan is the source for a majority of cultured saltwater pearls. The cultured freshwater pearl industry is increasing with a variety of shapes and colors created, although the typical shape is like a puffy rice grain, and color, white. For more pearl information, history, and lore, visit CW Jewelers. |  Dyed Black Cultured Pearl. Image taken from the former |
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Return to the Syllabus or choose another gemstone below.
| Alexandrite | Amber | Amethyst | Chalcedony | Diamond | Emerald |
| Garnet | Jade | Malachite | Opal | Pearl | Peridot |
| Ruby | Sapphire | Tanzanite | Topaz | Tourmaline | Turquoise |
This page originates from the Earth Science department for the use and benefit of students enrolled at Emporia State University. For more information contact the course instructor, S. W. Aber, e-mail: saber@emporia.edu Thanks for visiting! Webpage created: November 15, 2000; last update: January 8, 2007.
Copyright 1999-2007 Susan Ward Aber. All rights reserved.