Persian Gulf Pearl History
Since the past, persian Gulf was one of the greatest source for fine and perfect pearls. Persian Gulf oysters (Pinctada margaritifera, Pinctada radiata) were collected for their mother-of-pearl, which was used as an inlay material in ancient Egypt as early as the sixth dynasty (c 3200 BC). The pearls of the Persian Gulf were natural created and collected by breath-hold divers. The secret to the special luster of Gulf pearls probably derived from the unique mixture of sweet and salt water around the island. Unfortunately, The natural pearl industry of the Persian Gulf ended abruptly in the early 1930's with the discovery of large deposits of oil.
Figure1. Persian Gulf. Great source of wild-natural pearls
Most of the gemstone have a mineral and inorganic structure, but Since pearls structure comes from the organism, pearls originally have an organic structure. Inorganic gems are crystalline, while organic gems are non-crystalline so described as amorphous, or without the crystalline form. In the nature, pearls are found in black, white, pink and shades of other colors. Pearls have a distinctive concentric structure, and they have a specific gravity about 2.6-2.8. The main source of pearls are salt-water, such as persian Gulf, Japan, South Sea and Indian Ocean and fresh-water source and rivers including Mississippi and Amazon river. Pearls chemical composition are mainly calcium carbonate
(about 82%-86%), conchiolin and 2-4% of water (CaCO3 and H2O). Pearls are sensitive to heat and chemical materials. The bivalve of oysters are the best source for growing pearls. A good stimulus (usually a sand particle) is necessary for pearls production. The stimulus will place inside the oyster or mollusk, and oyster's defense mechanism starts secreting a calcium carbonate coating material to coat the irritate. Layer upon layer of this coating is deposited on the irritant until a lustrous pearl forms. It takes about 4-6 weeks for oyster to secrete the calcium carbonate layer around the particle nucleophile, and oyster can keep the pearl for several years. Pearls lifetime is between 100-150 years in the suitable situation and protected environment.
Figure2. Calcium carbonate structure
The Nacre and mother of pearls:
Pearls and mother of pearl are both made from nacre, an organic-inorganic nanocomposite material. It is strong, resilient, and iridescent, and it has high fracture-resistance. Nacre is secreted by some mollusks, including pearl oyster shells and freshwater pearl mussels. It is composed of plates of calcium carbonate (CaCO3), which can range from 300 to 1500 nm thick, and organic material that appears to act as a glue. For years, scientists believed that the strength and resilience of nacre stemmed from layers of calcium carbonate crystallized over preformed layers of chitin (a common scaffolding material found in many insect shells). Studies with scanning electron microscopes suggest that, rather than being laid down in sheets, the calcium carbonate is laid down in a brick-like structure scattered through an organic matrix. The presence of chitin is disputed, and many believe that the organic material is conchiolin. Because of its unusual properties, especially its extremely high fracture-resistance, the composition of this material is still being studied.
Figure 3. Structure of Nacre
Why is nacre iridescent?
Nacre is iridescent because the thickness of the calcium carbonate layer is similar to the wavelength of light. Light reflected from the outer surface is, therefore, able to interfere with light reflected from the inner surface. Constructive and destructive interference of different wavelengths of light produces different colors when viewed at different angles of incident light. Recently, research has demonstrated that diffraction plays a major role in creating iridescent color effects in mollusk shells. By...
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