National Museum of Natural History

Learn how mollusks make these shiny gems and how this biological process will change as the earth’s seas warm up

Most gems come from the inside of the earth and are made after millions of years of pressure and heat. But pearls-the most famous biological gems-come from the intestines of mollusks.

"Pearl is the word we use to describe the shiny works produced by mollusks. If the debris gets stuck in the mollusk and cannot be washed away, they will wrap the debris with their own mother-of-pearl or shell material," Smith Said Gabriela Farfan, curator of gems and minerals at the Forest Society, environmental mineralogist and curator of Coralyn W. Whitney. National Museum of Natural History.

Although all molluscs, including oysters, mussels and clams, are technically capable of making pearls, only some saltwater clams and freshwater mussels are used for commercial cultivation of gem-quality pearls.

"Only certain mollusk groups use a substance called nacre, which gives gem-quality pearls a milky white luster," said Chris Meyer, a marine invertebrate zoologist and museum curator of molluscs.

By collecting and analyzing nacreous pearls, scientists can learn more about how mollusks make these shiny gems and how this biological process will change as the earth's seas warm.

Mollusks make pearls to prevent irritants from invading their soft tissues. They do this by seeping through layer after layer of shell material. For some animals, this material is nacre or mother-of-pearl.

"What the animal does is wrap a skin around the invader, like a grain of sand or a parasite," Meyer said.

Nacre is an outer skin that gives pearls a pearly luster. But there is another reason for its particularity. The formulation of this material is made of organic secretions and a carbon-based mineral called aragonite, making it exceptionally strong.

"Its mineral and organic parts are combined like bricks and mortar," Farfan said.

This physical process can be traced back at least 200 million years in the fossil record, but natural pearls are extremely rare. Therefore, today people cultivate pearls to make more money for the gem market.

"The industry knows how to manipulate pearl production, which leads to all these pearl farms," ​​Meyer said.

Cultured or cultured pearls are usually smooth and spherical because of the way they are made.

"In essence, pearl farmers are very careful to insert a small bead made from a shell into the mollusk. Then they gently put the mollusk back into the ocean or lake and let it last for two to five years. Grow a pearl for harvest in the future," Farfan said.

Because the culture process is very effective, cultured pearls are easier to obtain than natural pearls. Therefore, their value is not rarity, but comes from their symmetry and luster.

"What really sets them apart is the gemologist's abilities," Meyer said. "For earrings, for example, the key is how well the pearls match in size and shape."

Although pearl farming is currently booming, like many other aquaculture industries threatened by climate change, it faces an uncertain future.

Global water temperatures are rising and local habitats are changing, both of which affect mollusks and may threaten all types of pearl manufacturing.

"Molluscs have the best temperature and environmental range. Just like you and me, their body functions perform their best in these places," said Stewart Edie, marine paleontologist and curator of bivalve fossils at the museum. ) Say. "Global warming will change these ranges and put animals under stress, so we need to study how this stress affects the energy trade-offs these animals must make."

Species known for making gem-quality pearls may begin to reuse their energy to maintain other biological needs. For example, the shells of saltwater mollusks are weakened by ocean acidification. These animals may need to invest more nacre to repair their dissolving shell, which means fewer irritants are captured.

"This is not a question of whether ocean acidification will affect seawater pearls, but how much ocean acidification will affect seawater pearls," Farfan said. "Furthermore, ocean acidification is just one of the major issues facing all mollusks and pearls. There are also hurricanes. , Water quality and pollution issues, etc."

But by examining pearls, researchers can see how mollusks respond to environmental fluctuations.

“By using pearls as mineral “time capsules”, we can understand how the environment around mollusks affects pearls and go back to better understand environmental changes,” Farfan said.

Now, she and other scientists in the museum are studying pearls in fresh and salt water bodies to learn more about how their mineralogy changes with the temperature and environment of the seasons and years.

Their findings can help them predict the fate of pearls and mollusks in the future.

"This will provide us with important information about how the environment affects these very amazing gems," Farfan said.

Related stories: Meet the coral reef experts who collect environmental time capsules and ask Smithsonian’s latest mollusk experts how biominerals become a stepping stone for climate change research and how the world’s largest aquamarine was born. Learn about the guardians of hope diamonds

Abigail Eisenstadt is a communications assistant at the Smithsonian’s National Museum of Natural History. She brings science to the public through the museum’s communications and public affairs office, where she tracks media reports, coordinates filming activities, and writes for the museum’s blog, Smithsonian Voice. Abigail received a master's degree in scientific journalism from Boston University. In her free time, she is either outdoors or in the kitchen. 

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