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article imageMystery of the spherical shape of pearls revealed

By Tim Sandle     Jun 21, 2013 in Environment
Scientists have revealed how pearls form into the most perfectly spherical large objects in nature. Pearls are produced by living shelled molluscs.
Arguably, pearls have the most perfectly spherical, or ball-like, shape among all the objects in nature that are visible without a microscope.
Pearls are made of calcium carbonate as nacre (mother of pearl) and other liquids accumulate around grains of sand or other foreign objects inside certain oysters and other shellfish. Nacre is the iridescent coating that is found on the inside of some molluscs and on the outer coating of pearls.
The way that pearls form perfect spheres is because developing pearls having a saw-toothed, or ratchet-like, surface. This texture generates forces that make the pearl turn inside the oyster's tissues in response to movements in the environment. The result of this rotation is a spherical build-up of nacre and other textures and the ‘perfect sphere’.
The rarest pearls of all are Queen Conch pearls. These pearls are formed by concentric layers of fibrous crystals, and this layering often produces the desired flame structure, which is characteristic of conch pearls.
The findings have been published in ACS' journal Langmuir. The reference is:
Julyan H. E. Cartwright, Antonio G. Checa, Marthe Rousseau. Pearls Are Self-Organized Natural Ratchets. Langmuir, 2013
In addition, scientists have recently synthesized the strong, iridescent coating found on the inside of some molluscs, creating an artificial pearl.
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