Pearls: Scientists have discovered the secret behind pearls’ amazing symmetry

Interesting science

Researchers have been perplexed for decades about how oysters produce wonderfully symmetrical, perfectly round pearls around oddly shaped pieces of sand or detritus. According to a new study, oysters, mussels, and other molluscs employ a sophisticated method to develop diamonds that follow mathematical patterns seen throughout nature.

When an irritant becomes caught inside a mollusc, the animal defends itself by forming smooth layers of mineral and protein, collectively known as nacre, surrounding it. According to a study published in the Proceedings of the National Academy of Sciences on October 19, each subsequent layer of nacre produced over this asymmetrical core adapts perfectly to the ones before it, smoothing out flaws and resulting in a round pearl.

Laura Otter, a biogeochemist at the Australian National University in Canberra, explains, “Nacre is this extremely beautiful, iridescent, shining material that we see in the insides of some seashells or on the outside of pearls.”

Otter and her colleagues determined that a pearl’s symmetrical growth as it lays down layers of nacre is dependent on the snail balancing two basic talents. It corrects growth aberrations that arise as the pearl develops, preventing such differences from spreading across its many layers. Otherwise, the finished gem would be uneven.

The mollusc also controls the thickness of nacre layers, so if one layer is particularly thick, the following layers will be thinner in response (SN: 3/24/14). This allows the pearl to maintain a consistent average thickness across its thousands of layers, resulting in a flawlessly round and uniform appearance. A pearl could resemble stratified sedimentary rock if it isn’t constantly polished, accentuating microscopic flaws that detract from its spherical shape.

Keshi pearls were harvested from Akoya pearl oysters (Pinctada imbricata fucata) in a coastal pearl farm in eastern Australia. They sliced the pearls into cross-sections with a diamond wire saw, then polished and studied the gems with Raman spectroscopy, a non-destructive technique that allowed researchers to determine the structure of the pearls. They counted 2,615 layers for one of the pearls included in the study, which were deposited over 548 days.

The study discovered that oscillations in the thicknesses of the pearls’ nacre layers reflect a phenomenon known as 1/f noise, or pink noise, in which seemingly random events are related. Creating nacre layers of various thicknesses may appear random in this scenario, but the thickness of prior layers determines it. In seismic activity, the same dynamic is at work: the rumbling of the ground appears random, but it is linked to past recent seismic activity. According to coauthor Robert Hovden, a materials scientist, and engineer at the University of Michigan in Ann Arbor, pink noise can be found in classical music and even when measuring heartbeats and brain activity. These occurrences “belong to a universal class of behaviour and physics”, according to Hovden.

According to Pupa Gilbert, a physicist investigating biomineralization at the University of Wisconsin–Madison who wasn’t involved with the study, “nacre self-cures and when a flaw emerges, it heals itself within a few [layers] without utilising an external scaffolding or template.” “Nacre is a material that is far more extraordinary than we had previously recognised.”