If you’ve ever gazed into the surface of a pearl, you may have wondered, “What is nacre?” That, my friend, is the reason pearls luster. Both iridescent and resilient, nacre (pronounced: nay-kər) is a composite material of organic and inorganic elements. Commonly called mother-of-pearl, nacre is produced by some mollusks to smoothly line an inner shell. This same material, wrapped tightly in concentric layers, gives rise to that most lustrous of gems: the pearl.
Nacre is composed of aragonite (a calcium carbonate mineral or CaCO3), shaped as hexagonal platelets and stacked à la brickwork in microscopically thin layers. The average size of a single platelet may be as small as 0.35 microns thick and 3.0 microns across, and only visible to the human eye through high magnification. Sheets of organic biopolymers such as lustrin, chitin and silk-like proteins bind each layer like glue or mortar, the combination of which creates a material that is incredibly strong and durable – comparative in strength to silicon.
What is nacre good for in nature? The simple answer is the protection of mollusk tissue. The saline ocean or freshwater river is a most hostile environment. Even the outer layers of mollusk shells are rough and abrasive, certainly not conducive to the delicate, sensitive folds of an oyster’s body. So the mollusk creates for itself a safe, smooth haven to call home by lining the walls that touch its soft tissue with nacre. Epithelial cells within the mantle tissue secrete this nacre.
When a foreign body enters the mollusk, like a parasite or fabled grain of sand, the mollusk’s response is to form a pearl sac, or cyst that contains the irritation and facilitates the healing process. Nacre is called upon again, this time wrapping around the intruder to soothe and minimize potential damage. In time and through successive layers, a pearl is formed. The process, called encystation, continues as long as the mollusk lives or until trauma expels the pearl from within.
Layers of nacre are added at irregular intervals, causing a delicate terrain much like a fingerprint that is unique to every pearl. The lightweight, transparent quality of nacre also gives rise to its chief characteristic: iridescence. The thickness of aragonite platelets is comparable to the wavelength of visible light, allowing light to pass through the layers and dance across its surface. Ridges and layers interfere constructively and destructively with different wavelengths at different vantage points, creating flashes of color and brilliant displays of orient on pearls. Each pearl bears witness to its own aurora borealis, as platelets act as tiny prisms refracting the spectrum of light.
Imitating nacre, as detailed by the Royal Society of Chemistry, is helping man create the next generation of harder, stronger, crack-proof ceramics. Indeed, smashing a seashell is no small endeavor, due in large part to the tightly stacked, interlocked composition of nacre. With a growing understanding of this arrangement of particulates, scientists are applying mollusk technology to industrial engineering. From building materials and bone replacements, to body armor and windshields, nacre has come a long way from the simple buttons and decorative inlays it once provided. Whether the simplest utilitarian application or the grandest of scientific ambitions, nacre is inspiring and changing the way we live our lives. Have time for one more? Caviar must always be dipped with a mother-of-pearl spoon, so as not to spoil the delicate flavors with silvery-metallic undertones.
Pearls, caviar and the advancement of science – what splendid reasons to don something lustrous!