How Pearls Obtain Their Remarkable Symmetry

Pearls form when a speck of sand, debris, or food particles are lodged inside a mollusk. The organism senses the wayward particle and begins to coat it with layers of aragonite and conchiolin, the same minerals and proteins mollusks use to build their shells. But scientists did not fully understand how mollusks form stunning and perfectly spherical pearls until now. Details of the study were published last month 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. 

For this study, researchers observed Keshi pearls taken from Akoya pearl oysters (Pinctada imbricata fucata) raised at an Australian coastal pearl farm. These pearls form naturally as opposed to bead-cultured pearls, which form when an artificial center is placed inside the mollusk, Science News reports. Using a diamond wire saw, the team cut each pearl into sections, polished them, and examined them under an electron microscope, reports Kate Mcalpine-Michigan for Futurity. One pearl formed an impressive 2,615 layers over the course of 548 days.

“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.

Another example of pink noise is seismic activity. Vibrations and rumbling in the ground during earthquakes are caused by previous seismic waves, per Science News. Other examples of "pink noise" can even be found in classical music, heartbeats, electricity, physics, and economic markets, Futurity reports.

“When you roll dice, for example, every roll is completely independent and disconnected from every other roll. But 1/f noise is different in that each event is linked,” Hovden explains to Futurity. “We can’t predict it, but we can see a structure in the chaos. And within that structure are complex mechanisms that enable a pearl’s thousands of layers of nacre to coalesce toward order and precision.”

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.  

Although nacre is made of calcium, proteins, and carbonate, the combination of these materials are 3,000 times tougher than each on is on their own. Researchers note that nacre’s durability and heat resistance could be used in the future for next-generation super materials in solar panels or other products that require tough and heat resistant material, per Science News.

“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,” Hovden says to Futurity.

Elizabeth Gamillo | | READ MORE

Elizabeth Gamillo is a correspondent for Smithsonian and a science journalist based in Milwaukee, Wisconsin. She has written for Science magazine as their 2018 AAAS Diverse Voices in Science Journalism Intern.