What are pearls?

I often get asked what pearls are and why bivalves make them. Pearls are biogenic gemstones. This means they are valuable rocks made not by inorganic crystallization within the earth, like most gemstones, but instead are produced by life! Interestingly, they are living rocks, composed of true minerals. When I talk about minerals, I mean a solid substance with a known chemical composition and crystal structure.

Pearls are specifically made mostly of a mineral called aragonite. Aragonite is a mineral made of atoms of calcium bound to an ion called carbonate. There are other minerals made from calcium carbonate, like calcite and vaterite. I’ll save those for another blog, and while those are present in small amounts in some pearls, the vast majority of the material is aragonite. The clam uses calcium carbonate to make pearls because it’s a conveniently available material: it’s also what they build their shells from!

Aragonite has a very specific geometric crystal structure at the molecular level, but zooming out slightly, it can be found in a tremendous variety of microfabrics. Like the fabric of our clothes, the shell is essentially “woven” by the bivalve with a certain texture at the cellular level. There are hundreds of types of fabrics, ranging from structures looking kind of like brickwork, to plywood, to actual long fibers of carbonate. But the most valuable pearls are made of a form of aragonite called nacre, which is also called “mother of pearl” for this reason.

The platy microstructure of nacre. See how the tablets are organized into interlocking columns! Source: Wikipedia

Nacre is a very special biomineral for many reasons. To humans, it’s precious because of its beautiful, complex iridescence and luster, which has attracted our eyes for thousands of years. But most clams aren’t making the material for its luster- they value its microfabric. Nacre is made of billions of tiny flattened tablets of aragonite, arranged in tall interlocking stacks. Each aragonite tablet also has little bridges joining it to the neighboring tablet, meaning they don’t easily slide out of place. The plated structure also aids the shell in staying together. Even when fractured, the shell can stay together as plates slide to lock into another shape!

Reviewing the various strengthening aspects of nacre, including the bridges that lock tablets together, the rough surfaces of the tablets that grip against each other, the organics that glue together tablets like mortar, and the tablets sliding into new locking orientations even if they break apart! From Zhao et al., 2018

Between the bricks of aragonite are a kind of mortar or scaffolding of protein, binding them all together. This protein scaffolding is extremely important to the overall material. Like the steel rebar in concrete, it strengthens the material, making it less brittle and therefore able to resist forces that might crush the clam’s shell, while still allowing for the material to be very thin. For us humans, those alternating layers of carbonate and protein act like thousands of layers of prisms, refracting the light into thousands of colors depending on the angle it is looked at, meaning that any light becomes a miniature rainbow when it passes through the structure of the nacre.

A snuff box made from a nacreous bivalve shell, at the Vienna Natural History Museum

This structure makes nacre a “premium” material for clams to build their shells from. It costs much more energy for a clam to make such an orderly microstructure and fill it with so much protein, up to 5% by weight, which is around 10-50 times how much organic material is found in other shell . Nacre is also more vulnerable to dissolving in the water surrounding the clam. For this reason, clams usually will only use nacre in the internal shell layer, isolated from the surrounding waters, using cheaper materials on the outside of the shell, or at least a protective sheath of protein on top (called periostracum). The nacre is present in mussels, oysters and other bivalves vulnerable to crushing pressures of waves as well as crushing predators like fish and birds. Nacre is like the bulletproof vest a clam uses to give itself a bit more powerful armor. Because pearls are made inside the shell, that’s why the most valuable pearls to us are made by oysters with a nacreous inner shell layer. The tropical pearl oysters (genus Pinctada) make some of the most valuable pearls, because it’s particularly rich in organic material and thus has a bright and complex lustre. Pearl oysters have extremely thin shells, which are strengthened by having a nacreous structure. They are

Why do bivalves make pearls? Pearls are essentially part of the bivalve’s immune system. If a piece of sand or debris got under your skin, your body would encase the intrusive object with scar tissue. Bivalves can do better than that, because they use the material of the shell to wrap around the object. Anything can be an intrusive object- a piece of sand, an infection, or even a parasite. For example, pearlfish are parasitic fish specialized to live inside the shells of clams. If they die in the shell, the clam will dutifully set to work encasing the fish in nacre, like Han Solo in carbonite!

A pearl oyster with a pearlfish wrapped in nacre against the inner shell. From the Natural History Museum London collection

Most natural pearls are irregularly shaped, so cultured pearls often use round plastic beads as the nucleus for the oyster to grow nacre around. The farmer wedges the oyster’s shell open, deposits the bead and leaves it for a period of time to allow the bivalve to deposit nacre. At harvest time, the oyster can be shucked to remove the pearl. Some experimental approaches even anaesthetize the oyster to remove the pearl, replace it with a new nucleus and repeat the process, potentially allowing for greater efficiency and humane harvesting, allowing the oyster stock to live for years!

So next time you see a pearl, you can understand that the craftsmanship of these wondrous objects is the result of millions of years of evolution, combined with thousands of years of human ingenuity. Moving forward, researchers are attempting to learn to imitate the structure and methods that clams use to make pearls, which could lead to all sorts of improvements in materials science! Clams again prove their skill in engineering. They have a 500 million year head start against us, but we can always learn!