Pearls Explained: The Biology and Physics of a Gem Made by a Living Animal

A pearl is a gem made by a living animal: a mollusk, such as an oyster or a freshwater mussel, coats an irritant or a piece of tissue with thin layers of nacre until a lustrous sphere forms. Nacre is built from microscopic crystals of calcium carbonate held together by protein, and the way light moves through those layers creates a pearl's glow. For readers in Palm Beach, where pearls have long been part of the dressing tradition, the biology makes them even more special to wear.

How does a mollusk make a pearl?

Inside every pearl producing mollusk is a thin organ called the mantle. Its cells normally build the inner lining of the shell. When something gets trapped inside the soft tissue, often a parasite or a tiny piece of the mantle itself, cells gather around it and form a pocket called a pearl sac.

The cells in that sac do what they always do: they secrete nacre, the same material that lines the shell. Layer after layer, over months and years, the pearl grows. It is a defensive response turned into something beautiful, and it happens without any cutting or polishing. Pearls are among the very few gems that come out of nature already finished.

What is nacre made of?

Nacre, often called mother of pearl, is a biological composite. Gemology references such as the Gemological Institute of America (GIA) describe it as:

• Aragonite platelets: microscopic, flat crystals of calcium carbonate, stacked in very thin layers.
• Conchiolin: an organic protein that glues the platelets together, a bit like mortar between bricks.
• Water and trace elements: small amounts that influence the final color and overtone.

This brick and mortar structure is why nacre has fascinated engineers. It is much tougher than calcium carbonate alone, because cracks have to wind their way around each tiny platelet instead of running straight through.

Why do pearls glow? The physics of luster and orient

Most gems sparkle because light bounces off cut facets. Pearls work differently. Light enters the translucent nacre, reflects from many stacked layers at slightly different depths, and comes back out. Those returning light waves overlap and interfere with each other, a little like the colors on a soap bubble.

The result is luster, the deep glow that seems to come from inside, and in fine pearls a soft rainbow shimmer called orient. Thicker, more even nacre generally gives stronger luster, which is one reason gemologists look so closely at nacre quality.

You can see that inner glow in the Large Pearl and Diamonds Earrings Maryon, which frame 10 mm natural freshwater pearls in 18K gold and natural diamonds. Next to the face, the pearl's soft luster and the diamonds' sharp sparkle show two completely different ways a gem can handle light.

Natural, cultured, freshwater and saltwater: what is the difference?

A natural pearl forms entirely by chance. A cultured pearl forms the same biological way, but a person starts the process by placing a small piece of mantle tissue, often with a bead nucleus, inside the mollusk. The animal then does all the real work, coating it in nacre.

Saltwater pearls grow in oysters in the sea, while freshwater pearls grow in mussels in lakes and rivers. Freshwater pearls often grow without a bead, which means they can be made of nacre almost all the way through, and they come in a lovely range of natural shapes and soft colors.

How hard is a pearl, and how should you care for it?

Pearls are soft compared with diamonds. They sit at roughly 2.5 to 4.5 on the Mohs hardness scale, while diamond sits at 10. Our guide on how hard a diamond is explains what those numbers mean. Because nacre is calcium carbonate, it is also sensitive to acids, perfume and cosmetics.

• Put pearls on last, after perfume, hairspray and sunscreen.
• Wipe them with a soft, slightly damp cloth after wearing.
• Keep them away from pool chlorine and harsh cleaners.
• Store them separately from diamonds and gold, which can scratch the surface.

For a ring, a protective setting matters. The Maryon Ring pairs freshwater pearls with conflict free diamonds in 18K gold, so it feels special but is best kept for moments when your hands are not doing heavy work.

Why do pearls and diamonds work so well together?

Pearls and diamonds are opposites in almost every scientific way. One is soft and organic, built by a living cell. The other is the hardest natural material, formed from carbon under enormous pressure deep in the Earth. Together they balance glow and fire.

The Pearls and Diamonds Maryon Necklace brings both into one pendant, in 18K recycled gold with a conflict free natural diamond and pearls on an 18 inch rolo chain. For wedding ideas, see our guide to pearl and diamond earrings for a wedding.

Margaret Krinetz Fine Jewelry, handmade in Tel Aviv and shipped to Palm Beach, comes from a studio where a background in biology meets goldsmithing, and pearls are where those two worlds meet most naturally.

Frequently asked questions

Can pearls be worn every day?

Pearl earrings and pendants can, with care. Rings see more knocks, so wear them more gently.

Are freshwater pearls real pearls?

Yes. They are made by freshwater mussels from the same nacre as other pearls.

How long does delivery to Palm Beach take?

Each piece is handmade to order in 4 to 6 weeks, then ships by DHL Express in 3 to 5 business days. Shipping is free on orders over $1,500.

Curious about a pearl piece made just for you? Get in touch with Margaret.