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.
—Sophie Bushwick
