How an oyster builds a perfectly round pearl

For centuries, researchers have wondered how oysters grow incredibly symmetrical and perfectly round pearls around irregularly shaped grains of sand or debris. Today, a team has shown that oysters, mussels, and other mollusks use a complex process to grow gemstones that follow mathematical rules seen throughout nature.

Pearls form when an irritant is trapped inside a mollusk, and the animal protects itself by building smooth layers of minerals and proteins – together called nacre – around it. Each new layer of mother-of-pearl built on this asymmetrical center matches precisely those that precede it, smoothing out irregularities to result in a round bead, according to an analysis published Oct. 19 in the Proceedings of the National Academy of Sciences.

“Mother of pearl is that incredibly beautiful, iridescent and shiny material that we see inside some seashells or outside pearls,” says Laura Otter, a biogeochemist at Australian National University in Canberra.

The symmetrical growth of a pearl when it deposits layers of nacre relies on the mollusk balancing two basic abilities, found Otter and his colleagues. It corrects growth aberrations that appear as the pearl is formed, preventing these variations from spreading to the many layers of the pearl. Otherwise, the resulting gem would be out of balance.

In addition, the mollusk modulates the thickness of the mother-of-pearl layers, so that if one layer is particularly thick, subsequent layers will be thinner in response (ND: 03/24/14). This helps the pearl maintain a similar average thickness over its thousands of layers so that it is perfectly round and even. Without this constant adjustment, a bead might look like layered sedimentary rock, amplifying small imperfections that detract from its spherical shape.

Researchers studied keshi pearls harvested from Akoya pearl oysters (Pinctada imbricata fucata) at a coastal pearl farm in eastern Australia. They used a diamond wire saw to cut the pearls into cross sections, then polished and examined the gemstones using Raman spectroscopy, a non-destructive technique that allowed them to characterize the structure of the pearls. For one of the beads featured in the article, they counted 2,615 layers, which were deposited in 548 days.

cross sectional image of a keshi pearl
This cross section of a keshi bead shows that the round gem is growing around a misshapen piece of debris.Jiseok Gim

Analysis revealed that fluctuations in the thicknesses of the nacre layers of the pearls exhibit a phenomenon known as 1 / f noise, or pink noise, in which events that appear to be random are actually connected. In this case, the formation of mother-of-pearl layers of different thicknesses may appear random, but in reality depends on the thickness of the previous layers. The same phenomenon is at work in seismic activity: the rumbling of the ground seems random, but is actually linked to a previous recent seismic activity. Pink noise also appears in classical music and even when monitoring heartbeats and brain activity, says co-author Robert Hovden, a scientist and materials engineer at the University of Michigan at Ann Arbor. These phenomena “belong to a universal class of behavior and physics,” says Hovden.

This is the first time that researchers have reported “that mother of pearl heals itself and when a defect occurs, it heals itself in a few minutes. [layers], without using scaffolding or external jigs, ”says Pupa Gilbert, a physicist studying biomineralization at the University of Wisconsin-Madison who was not involved in the study. “Mother of pearl is an even more remarkable material than we had appreciated before. “

Otter Notes: “These humble creatures make super light, super tough material so much easier and better than we do with all of our technology. “Composed only of calcium, carbonate and proteins, mother-of-pearl is” 3,000 times more resistant than the materials it is made of. “

This new understanding of pearls, Hovden adds, could inspire “the next generation of supermaterials,” such as more energy-efficient solar panels or rugged, heat-resistant materials optimized for use in spacecraft.

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