Pearls: Perfect Symmetry

How are pearls formed and how do they achieve such symmetrical perfection? Here's how.

Perfect pearl resting on the tip of a finger

Humans have been fascinated by these iridescent beauties since we first discovered them thousands of years ago. Even the ancient Egyptians and Romans were obsessed with these gems, designating them as a symbol of wealth and beauty. If only they knew what we do now, they would be even more amazed.

Pearls are created when an irritant, such as a grain of sand or debris, gets trapped inside a mollusk. As protection, a smooth layer of mineral and protein called nacre forms around it.

Scientists have long puzzled exactly how mollusks create these symmetrical wonders around such irregular shaped starting materials. Finally, a paper published in Proceedings of the National Academy of Sciences has linked this ability back to some complex mathematical rules that are seen throughout nature.

Scientists from the Australian National University in Canberra got to this conclusion by studying Akoya pearl oysters. A diamond wire cutter was used to chop the pearls into cross-sections, and analysis techniques were carried out to determine its inner structure.

The team found the pearl contained 2,615 layers, which were deposited over 548 days. 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. This phenomenon in nature is called 'pink noise', where events seem to be random but are really connected. Also, the team found the nacre self-heals when a defect is spotted, allowing for smoothing out of irregularities when they appear.

Only made from calcium, carbonate, and protein, the nacre is actually 3000 times stronger than the materials it is composed of! Laura Otter, author of this paper stated: “These humble creatures are making a super light and super tough material so much more easily and better than we do with all our technology.”

This strong nacre could inspire the next generation of super materials, with applications in impact and heat-resistant materials for spacecraft or in solar panels with increased energy efficiency.