It is unfortunately rare that people look at snails and oysters and think, “bad-ass.” The giant octopuses and squid get all the attention in family Mollusca, while the lowly slugs and clams are considered little more than animate pulpy goo which may or may not be enclosed by a crunchy shell. But gastropods and bivalves do something that octopuses cannot: secrete pearl. Unfortunately for these lesser molluscs, even pearls have become untrendy lately. You don’t see many young women wearing pearls; they’re something your mother or grandmother would wear, a fashion throwback for First Ladies and Daughters of the American Revolution, barely a step up from costume jewelry. While kings have had no problem rocking emeralds and rubies, pearls have long been seen as something effeminate, perhaps even wimpy. I’d say General George Patton put it best, when correcting a reporter about the white inlay of his pistol handles: “Ivory. Only a New Orleans pimp would carry a gun with Mother of Pearl grips.”
Little did Patton know that the animal that secretes Mother of Pearl might someday aid tomorrow’s war heroes.
Calcium carbonate is neat stuff. When you learn about Ancient Greece and the teacher writes the word “Parthenon” on the board with chalk, she’s using calcium carbonate, and the Parthenon itself is made of marble, which is metamorphosed limestone, which is calcium carbonate. You find it in the cave flowers and stalactites of sunless Carlsbad Caverns, in the haunting tufa structures of California’s Mono Lake, the coral cities of the Great Barrier Reef, and in your medicine cabinet as antacids. Of course, it makes up the shells of most marine molluscs, and when restructured as aragonite, one of its many polymorphs, it becomes nacre, or mother-of-pearl. Nacre helps smooth out a mollusc’s rough life, protecting it from being chafed by its own shell. A conventional misperception of pearls is that they’re made when an oyster secretes layers of nacre around an irritating piece of sand that’s gotten stuck in its folds. This does happen sometimes, but most often the irritating offender is a parasite. It’s a pretty horrifying way to go, when you think of it: A pesky parasite tries to get the best of a giant mollusc, but when it’s discovered, gets entombed in carbonite and turned into a highly-valued and dearly-bought jewel.
Nacre produces its signature iridescence because the hexagonal aragonite “bricks” that build it are close in size to the wavelengths of visible light. Structure is key to the molluscs’ tough and beautiful shells; when you carry your home with you and never leave it, it helps to know something about masonry. And to understand the genius of mollusc architecture, we turn to one of the greatest manufacturers of mother-of-pearl: the abalone.
Abalones are not oysters but large sea snails which munch on seaweed in the subtidal zones of the world. They have one strong foot which keeps them from carried away by the strong currents and occasional waves, and their shells bear signature respiratory holes for their gills. By most accounts, they are also delicious when cooked with butter; in Northern California, where I lived for a few years, they are so popular there is now a moratorium on hunting abalone south of San Francisco Bay, and north of it there are strict limitations on how you can catch them. (For example, divers may only hold their breath; no snorkeling allowed!) Of course, they’re also coveted for their shells, and it’s not hard to see why.
And here’s a New Zealand abalone, after polishing:
Abalones have good reason to build strong homes, because they share a condition that leaves them incredibly vulnerable: hemophilia. The slightest scratch, and they can bleed to death. Nacre is a tremendously durable substance, but the “pearly” part of nacre is only the outermost, light-refractive layer of the shell. It turns out that the bricks of calcium carbonate which make up 95% of the abalone’s shell are arranged in the most efficient way conceivable, making it the strongest material for its weight in the animal kingdom. It is so strong that military researchers are copying its structure to build the next generation of body armor. Keep in mind, we’re talking about a shell that is 95% chalk.
The calcium carbonate tiles, each about 0.001th the width of a strand of human hair, are cemented vertically with a gluey protein. When struck, the tiles slide apart from each other laterally while never fully breaking away from their columns, absorbing the impact by dissipating the force. Cracks are quickly filled in with nacre. According to scientists, there has never been a more resilient structure made by a natural organism. Over the course of mollusc evolution, every shape and arrangement of chalk tiles may have been tried, and this particular structure proved the best of any theoretically possible: the perfect shell. It may not stop a bullet as it is (sorry, would-be abalone tough guys), but if we can recreate the structure using a material stronger than, say, Tums, it will prove to be bullet proof. Suddenly, it’s going to be a lot more manly to wear pearls.