A Pearl is a hard, rounded secretion formed inside the shell of certain bivalve mollusks. As a response to an irritating object inside the shell, the mollusk will deposit layers of calcium carbonate (CaCO3) in the form of the minerals aragonite or calcite (crystalline forms of calcium carbonate) held together by an organic horn-like compound called conchiolin.
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 are thinner and more numerous. The iridescence which 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, or black. Black pearls, because of their rarity, are often highly valued.
Almost all pearls used for jewelry nowadays are cultured by planting a core into oysters. The pearls are usually harvested two years after the planting. This mariculture process was first developed by the Japanese. The original Japanese cultured pearls are produced by a species of small oysters no bigger than 6 to 7 cm in size, hence Japanese pearls larger than 10mm in diameter are extremely rare and highly priced. In the past couple of decades, cultured pearls are produced with larger oysters in the south Pacific. South sea pearls are characterized by their large size and silvery color, 14mm in diameter are not uncommon. Recently (in the 1990s), the Japanese also invested in producing cultured pearls with fresh (brackish?) water mollusks in the region of Shanghai, China. Freshwater pearls are characterized by the reflection of rainrow color in the luster.
The value of the pearls in jewelry are often judged by the luster, color, size, lack of surface flaw, symetry that are appropriate for the type of pearl under consideration. Amongst those attributes, luster is the most important differentiator of pearl quality according to jewelers. For example, a small Japanese pearl is often valued higher than a bigger south sea pearl. Large perfectly round pearls are rare and highly valued and reserved for making rings. Teardrop shaped pearls are often used in pendants. Irregular shaped pearls are often used in necklaces.