Why Do Oysters Make Pearls?

The creation of pearls has been romanticized, with many believing that oysters produce them when a grain of sand enters their shell, but that’s not always true.

May 14, 2024By Monika Dimitrovska
why do oysters make pearls

Oysters, clams, and mussels have been around for 550 million years. These creatures don’t have backbones but soft bodies inside protective shells, and as you know, oysters produce pearls. But why do oysters create pearls in the first place? The short answer is pain and discomfort.

When something hurts oysters, they coat it with layers of a shiny substance called nacre. This acts as a shield for the oyster. Over time, these layers build up and turn into a pearl. Let’s learn all the details!

Why Do Oysters Make Pearls?

oyster shell pearl beach
Image credit: Elias from Pixabay

Pearls, unlike most gems, come from animals that live in seashells, like oysters. Most pearls come from oysters in freshwater or saltwater.

Oysters aren’t the only animals that make pearls; sometimes clams and mussels join in, but it’s less common.

To understand how oysters make pearls, let’s explore what’s beneath the shell.

The shell is made of two parts joined by a stretchy ligament. Inside, there’s a mouth (palps), stomach, heart, intestines, gills, anus, abductor muscle, and a mantle.

As oysters grow, their shells grow, too. This happens because the mantle, a thin layer inside the shell, adds new material to the shell’s edges. The mantle takes minerals from the water and turns them into what the shell needs.

The shell mostly consists of calcium carbonate minerals and a natural protein made by the mantle. Calcium carbonate, like chalk, makes the shell strong.

Now that we know where seashells come from, let’s explore their layers.

The shell has three layers: the outer proteinaceous periosteum, the middle prismatic layer, and the inner nacre layer. The nacre layer is shiny and colorful, often called the “pearl layer” or “mother of pearl.” It’s used to make decorative items like buttons.

Interestingly, the same shiny nacre inside the shell forms pearls, too.

What is The Purpose of Pearls in Oysters?

pacific oyster
Image credit: Wikipedia

The aquatic world is truly interesting and somewhat bizarre. Male seahorses give birth, while oysters craft pearls when something foreign gets inside them and irritates them.

To protect itself, the oyster covers the irritant with layers of nacre, which also makes the shell of the oyster, as we already explained.

Many think pearls form around grains of sand, but that’s not always true.

Most pearls form because of parasitic organisms like drill worms that invade the oyster’s shell. They burrow through the hard shell and trigger the mantle. This makes the oyster create a barrier around the invader, which eventually becomes a pearl.

pearl inside oyster
Image credit: günter from Pixabay

Some oysters, like Pinctada mazatlanica, can make three or four thin layers of nacre every day. Each layer is super thin, about a thousandth of a millimeter. It takes around 24 months for osters to make a natural pearl as big as 5 millimeters, which is about the height of 20 stacked playing cards.

As you can see, these oysters have some pretty unique traits. Still, they’ve got nothing on the weirdest deep sea creatures.

Additionally, pearls come in different sizes and shapes. Some have a perfectly rounded shape, while others aren’t that even. And expectedly, the round ones usually hold more value.

Natural pearls can be really small, like 0.03 inches, or a bit bigger, around 0.27 inches on average. Pearls that get to be 0.39 inches or more hold more value because they’re rare. Usually, the bigger a natural pearl is, the more valuable it is.

Cultured Pearl vs. Natural Pearl

pearls different shapes
Image credit: shraga kopstein from Unsplash

You can also find cultured pearls on the market (made with human help). They’re not as rare or expensive as natural pearls, but they’re still real.

Pearl farmers open the oyster shell and make a small cut in the mantle tissue. Then, they place small irritants under the mantle, and the oyster starts making a pearl with nacre.

Sometimes, they use a grafting process, putting a pearl nucleus into the oyster to start the pearl growth. Usually, just cutting the mantle is enough for the oyster to start making a pearl, even without adding irritants.

How Long Does it Take for an Oyster to Make a Pearl?

white pearl inside oyster
Image credit: Marin Tulard from Unsplash

For natural pearls, it can take around six months for some pearls to form. But if the pearl is bigger, it might take up to four years. Bigger pearls usually have a higher value because they take longer to grow.

For cultured pearls, once the irritant is put inside the oyster, it takes about six months to form a pearl. However, if the pearl is going to be larger, it can take up to four years to develop fully. This is why bigger pearls often cost more.

Can You Remove a Pearl Without Killing the Oyster?

person removing pearl from oyster
Image credit: Thomas John from Unsplash

Oysters enrich the ecosystem in many ways; hence, we shouldn’t encourage the pain and killing of oysters by buying pearls. While it’s possible to remove the pearl without harming the oyster, this is rarely the case because most harvesters don’t care about the oysters.

Additionally, not all oysters survive this, and most end up killed, which is why pearls aren’t ethical. Some pearl farmers take pearls out without hurting the oyster, but lots of them don’t make it. They grow pearls on farms where they take care of thousands of oysters for a few years until they’re ready.

How Rare is it to Find a Pearl in an Oyster?

black pearl
Image credit: Wikipedia

Finding a pearl is really rare because not all oysters produce pearls. Only about one in every ten thousand oysters does. And out of those, only about one in a million is really good.

As we already mentioned, pearls come in different colors, and if you find a black one, consider yourself lucky because black pearls aren’t common. They come from special oysters with black shells.

What is Inside a Pearl?

non bead cultured pearl construction
Image credit: Wikipedia

Inside a pearl, you’ll find calcium carbonate, which is a mineral commonly found in shells. It’s organized in layers of small crystals, resembling growth rings in a tree. While we often think of pearls as perfectly round and smooth, they can also take on various shapes, known as baroque pearls.

Monika Dimitrovska
By Monika Dimitrovska

Monika is a pet enthusiast and seasoned copywriter with a tech degree. She loves writing, but her heart belongs to her two mixed dogs, Buba and Bono, a mother-son duo. Bono’s siblings found loving homes, sparking Monika’s advocacy for neutering and deepening her curiosity about animal care.

But Monika’s pet family doesn’t end there. She also has two cockatiels and two rescue cats, proving her home is a haven for creatures big and small.