How an Oyster Forms a Perfectly Symmetrical Pearl

It's a level of symmetry that outstrips everything else in the natural world, with the exception of individual atoms.

“We humans, with all our access to technology, can’t make something with a nanoscale architecture as intricate as a pearl,” said Robert Hovden, University of Michigan assistant professor of materials science and engineering. “So we can learn a lot by studying how pearls go from disordered nothingness to this remarkably symmetrical structure.”


The University of Michigan's study found that a pearl’s symmetry becomes more and more precise as it builds, answering centuries-old questions about how the disorder at its center becomes a sort of perfection.


The answer? Layers of nacre, the iridescent and extremely durable organic-inorganic composite that also makes up the shells of oysters and other mollusks, build on a shard of aragonite that surrounds an organic center. The layers, which make up more than 90 percent of a pearl’s volume, become progressively thinner and more closely matched as they build outward from the center.


The clever trick is that oysters maintain the perfect symmetry of their pearls by adjusting the thickness of each layer of nacre. If one layer is thicker, the next tends to be thinner, and vice versa.


“These thin, smooth layers of nacre look a little like bed sheets, with organic matter in between,” Hovden said. “There’s interaction between each layer, and we hypothesize that that interaction is what enables the system to correct as it goes along.”


Hovden says the study’s findings, published in Proceedings of the National Academy of Sciences, could help inform next-generation materials with precisely layered nanoscale architecture.


“When we build something like a brick building, we can build in periodicity through careful planning and measuring and templating,” he said. “Mollusks can achieve similar results on the nanoscale by using a different strategy. So we have a lot to learn from them, and that knowledge could help us make stronger, lighter materials in the future.”

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