June Birthstone - Pearl

Physical and Chemical Properties

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Chemical composition

Pearl is unique among the birthstones inasmuch as it is of organic origin. All other birthstones are minerals, inorganic solid substances with crystalline structures and fixed chemical compositions that vary only within rigid limits. Pearl are made up of little overlapping platelets of the mineral aragonite, a calcium carbonate that crystallizes in the orthorhombic system. In cross section a pearl will appear to have concentric, smooth layers, but magnification will show these layers have an imbricate (brick wall-like) structure. These tiny plates are held together by an organic cementing agent called conchiolin. The pearl derives its iridescence from the diffraction and interference of white light that is caused by the tiny overlapping platelets of calcium carbonate. The iridescence or orient of the pearl is a function of the numbers and thickness of these platelets. Mother of pearl or nacre forms on the inner walls or inner surfaces of the mollusk shell. Mother of pearl differs from pearl inasmuch as it is part of the mollusk shell whereas the pearl has become a separate entity from the shell.

Colors

In America, the most commonly bought pearls are white with pink tones. But pearls may also be tan, yellow, brown, gray, or black with a purple or green luster. The color of this june birthstone depends on the color and chemicals found in the shell of the mollusk.Most pearls are spherical, but irregularly shaped specimens are called "baroque."

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