Mother-of-Pearl

Nacre, also known as mother of pearl or sadaf, is a naturally occuring organic-inorganic composite. more...

It is formed of layers of calcium carbonate (CaCO₃) platelets (crystals), in the form of aragonite and conchiolin (a scleroprotein), separated by elastic biopolymers (such as chitin and lustrin). This mixture of hard and elastic domains makes the material strong and resilient. Strength and resilience is also likely to be due to the offset or "brickwork" arrangement of the platelets, which inhibits transverse crack propagation.

The iridescent appearance of the nacre is due to the fact that the thickness of the aragonite platelets are about 0.5 micrometres, which is comparable to the wavelength of visible light. This results in constructive and destructive interference of different wavelengths of light, resulting in different colors of light being reflected at different viewing angles.

Nacre is secreted by the epithelial cells of the mantle tissue of certain species of mollusk. In these mollusks, nacre is continually deposited onto the inner surface of the animal's shell (the iridescent nacreous layer, commonly known as mother of pearl), both as a means to smooth the shell itself and as a defense against parasitic organisms and damaging detritus.

The iridescent inner layer is considered highly attractive by many cultures and is often used in making jewelery or as inlays in wood furniture and guitars.

When a mollusk is invaded by a parasite or is irritated by a foreign object that the animal cannot eject, a process known as encystation entombs the offending entity in successive, concentric layers of nacre. This process eventually forms what we call pearls and continues for as long as the mollusk lives.

Chief sources are the pearl oyster, found in warm and tropical seas, primarily in Asia; freshwater pearl mussels, which live in many rivers of the United States, Europe, and Asia; and the abalone of California, Japan, and other Pacific regions.

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