How Pearls Form into Perfect Spheres

6mm round USA natural freshwater pearl

6mm round USA natural freshwater pearl

“Pearls, the most flawless and highly prized of them, are perhaps the most perfectly spherical macroscopic bodies in the biological world. How are they so round?”

So begin Julyan Cartwright at the University of Granada in Spain and a few pals in a paper that gives an interesting answer to this question. Such a mechanism must not only explain the near spherical perfection but also why pearls form other shapes too, such as drop-shapes, which have rotational symmetry but non-spherical shape, and so-called baroque pearls that have no symmetry.

The answer turns out to be based on a relatively simple effect. Cartwright and co say the surface of a pearl has a ratchet-like texture. This generates a force that tends to turn the pearl as it grows in the presence of random jostling from the environment. “Pearl rotation is a self-organized phenomenon caused and sustained by physical forces from the growth fronts,” they say. “Rotating pearls are a — perhaps unique — example of a natural ratchet.”

In the absence of other forces, this rotational process causes the pearl to become spherical. However, small defects in the shape of the pearl can easily distort the process so that certain rotational symmetries end up being preferred. The result in that case is that the pearl becomes non-spherical but maintains a rotational symmetry, forming a drop-shape for example.

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