How Pearls Obtain Symmetry

Scientists did not fully understand how mollusks form stunning and perfectly spherical pearls until now. Details of the study were published in the Proceedings of the National Academy of Sciences.

Researchers have now found that mollusks use a complex layering process that follows mathematical rules seen throughout the world, reports Rachel Crowell for Science News. Layers of the aragonite and conchiolin are called nacre, and after each layer forms, mollusks will adjust each sheet to maintain its symmetry. If one layer of the pearl’s nacre is thinner, the next layer will be thicker to balance out irregularities, over time creating a smooth, uniform pearl that isn’t lopsided. The process is then repeated until thousands of layers of nacre from the gem.

“These thin, smooth layers of nacre look a little like bedsheets, with organic matter in between,” study author Robert Hovden, a materials science expert and engineer at the University of Michigan in Ann Arbor, tells Futurity. “There’s interaction between each layer, and we hypothesize that that interaction is what enables the system to correct as it goes along.”

After observing the pearls under the microscope, the team found that the interactions between each layer and its thickness follow a phenomenon known as 1/f noise, or pink noise. In pink noise, events that appear random are actually connected, per Science News. In the case of pearls, the formation of each mineral and protein sheet and their thickness may seem random, but the thickness of each previous layer determines each new layer’s shape.

While pearls lack carefully planned symmetry that keeps brick buildings in order, pearls will maintain symmetry for 20 layers at a time, which is enough to accumulate consistency over its thousands of layers. In a way, the pearl “self-heals” when defects arise without using external scaffolding as a template, comments Pupa Gilbert, a physicist at the University of Wisconsin-Madison who was not involved with the study, to Science News.

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