Most gems come from the bowels of the Earth, made by pressure and heat over millions of years. But pearls — the most famous biological gems — come from the bowels of mollusks.
“Pearl is a word we use for a shiny creation that a mollusk produces. If debris gets stuck in a mollusk and they can’t flush it out, they coat this debris in their own mother of pearl or shell material,” said Gabriela Farfan, environmental mineralogist and Coralyn W. Whitney curator of gems and minerals at the Smithsonian’s National Museum of Natural History.
While all mollusks, including oysters, mussels, and clams can technically make pearls, only some saltwater clams and freshwater mussels are used to commercially grow cultured gem-grade pearls.
“Only certain mollusk groups use a substance, called nacre, which gives gem-quality pearls their opalescent sheen,” said Chris Meyer, a marine invertebrate zoologist and curator of mollusks at the museum.
By collecting and analyzing nacreous pearls, scientists can learn more about how mollusks create these shiny gems and how that biological process could change as Earth’s waters warm.
Mollusks make pearls as a protection against irritants that sneak into their soft tissue. They do so by exuding layer upon layer of shell material. For some animals, this material is nacre, or mother of pearl.
“All the animal is doing is putting a rind around an intruder, like a grain of sand or parasite,” said Meyer.
Nacre is a type of rind that gives pearls their pearly sheen. But it’s also special for another reason. The material’s recipe, made of organic secretions with a carbon-based mineral known as aragonite, makes it exceptionally strong.
“It’s mineral and organic parts go together like bricks and mortar,” said Farfan.
This brick-and-mortar process dates to at least 200 million years in the fossil record, but natural pearls are incredibly rare. So, people today farm pearls to make more for the gem market.
“There’s this industry that knows how to manipulate pearl production, which has led to all these pearl farms,” said Meyer.
Seeding an industry
Farmed, or cultured, pearls are usually smooth and spherical, because of how they’re made.
“Essentially, the pearl farmers very carefully insert a little bead made of shell into the mollusk. Then they gently put the mollusk back into the ocean or a lake and let it grow a pearl over the course of two to five years for harvesting later on,” said Farfan.
Since the farming process is so effective, cultured pearls are more widely available than their natural counterparts. So, instead of rarity, their value comes from their symmetry and shine.
“It’s really the gemologist’s capacity to match them that makes them something really special,” said Meyer. “For example, with earrings, it’s about how closely the pearls are matching in size and shape.”
Although pearl farming is thriving currently, it faces an uncertain future just like many other aquatic industries threatened by climate change.
Pearls in peril
Global water temperatures are rising, and local habitats are changing, both of which will affect mollusks and could threaten all types of pearl making.
“Mollusks have optimum temperature and environmental ranges, like you and me, where their bodily functions work best,” said Stewart Edie, marine paleobiologist and curator of fossil bivalves at the museum. “Global warming will shift these ranges, placing the animals under stress, and so we need to study how that stress affects the energy trade-offs that these animals will have to make.”
Species known for creating gem-quality pearls may start redirecting their energy to sustaining other biological needs. For example, saltwater mollusks’ shells are weakening from ocean acidification. These animals might need to devote more nacre to repair their dissolving shells, meaning less to trap irritants.
“It’s not a question of if, but how much will saltwater pearls be impacted by ocean acidification,” said Farfan, “And, ocean acidification is only one of the big issues facing all mollusks and pearls. There are also hurricanes, issues with water quality and pollution, and so on.”
But by examining pearls, researchers can see how mollusks respond to environmental fluctuations.
“By using pearls as mineral “time capsules,” we can look at how the environment around the mollusk has influenced the pearl and backtrack to get a better picture of environmental change,” said Farfan.
Right now, she and other scientists at the museum are studying pearls from fresh and saltwater bodies to learn more about how their mineralogy shifts under changing temperatures and environments over the seasons and years.
What they find could help them predict the fate of pearls and mollusks in the future.
“It’s going to give us important information as to how environments impact these very amazing gems,” said Farfan.