4, 000 years ago, a Chinese discovered the first pearl in oysters. The inner of an oyster' shell is covered with a smooth shiny coating which is called
mother-of-pearl. When oyster is opening its shell for eating in the sea-bed, an irritant, such as a grain of sand, goes in suddenly, the oyster immediately takes action by closing its shell, then the sand or irritant gets caught inside the oyster's shell. To protect the body, oyster coats the irritant with its secrets layers of mother-of-pearl. Finally, the coated grain of sand becomes a pearl.
The substance of pearl and oyster is the same. Its shape is changeable, but generally can be divided into two types: regular shape and irregular shape. The regular shape consists of round, semi-circular, bun-shape, oval, long circle shape, clavate, flat circular, pear shape and so on. The shape of irregulars is determined by its core, the pearl with core is changed with the shape of core, in none core pearls, in except of those describable shapes, the rest all belong to irregular shape. The hardness of pearl is 3.5-4.5, which is higher than copper's 3 and similar to iron's 4. The specific gravity is about 2.70; quality pearl is much bigger in 2.65-2.68, while the poor one is smaller. The index of pearl elasticity is determined by its inner Keratin content in the shell. When a pearl free fall on the glass from a 70cm height, normally it can bounce to 37cm height, the quality one can jump even more higher.
To judge a pearl's quality, color and luster should be the most important symbol. The color and luster is produced by the reflected light from the pearl's outer and inner; in addition, it is also related with the body color which is in the pearl's color and metal element. According to the spectrometry, the pearls in golden yellow and cream color contains more copper and silver; pink and flesh color with more sodium and zinc; white and pink with more manganese. Therefore, the color and luster of
pearl can be divided into white series, pink series, yellow series and blue series. A round pearl's color and luster is created from spherical surface, it is different from the category, status and background of light source, and changes with the observation angle.
Thus, the color and chemical element in water is closed connected with the species and amount of plankton, different water and depth of
pearl can produce different quality and color pearls, with the growing of oyster, the older the oyster is, the deeper color the pearl will be. If the shell of oyster is yellow, then the pearl it produces is mainly in yellow series.
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How do oysters produce pearls?
