Flawless pearls are among the most symmetrical spheres with biological origins. But how do they get so round? Turns out they turn.
Pearls form when mollusks such as oysters create so-called pearl sacs around intrusive pieces of grit. The sac coats the irritant with layers of smooth nacre, also known as mother-of-pearl. The growing pearl rotates itself, which allows the nacre to deposit evenly over its surface.
By examining pearls under a fluorescence and a scanning electron microscope, researchers discovered that the surface actually has a saw-tooth texture. As the mollusk moves, the pearl is jostled to the next tiny tooth. The work is published in the journal Langmuir. [Julyan H. E. Cartwright, Antonio G. Checa, and Marthe Rousseau, Pearls Are Self-Organized Natural Ratchets]
A pearl's motion influences its nacre coverage, and thus its final shape. Depending on its surface pattern, it might turn in a single direction to create a drop or ring, or rotate more freely to form a sphere. If a defect prevents this motion, the final product will be shapeless. The resulting asymmetrical pearl is doomed to be booed. Roundly.
[The above text is a transcript of this podcast.]