How Do Pearls Form?

Every pearl, natural or cultured, is grown by a living mollusk. The sections below follow a pearl from the first cells to the finished layers, and show how pearl farms copy the process.

What a pearl is made of#

A pearl is made of the same material as the lining of the shell it grew in. The mantle, the tissue that builds the shell, also makes pearls, and pearls have the same composition as shell nacre, with calcium carbonate as the main ingredient.¹

Nacre forms in layers of microscopic aragonite platelets, bound together by an organic material called conchiolin. That layered structure is what gives a pearl its orient and overtone.²

Not every pearl has nacre. Conch, clam and scallop pearls, for example, are natural pearls without a nacreous surface.²

Natural pearl

• Starts by accident, with no human help
• Nacre layers reach a tiny center

Bead-cultured pearl

• A shell bead and a graft go into the gonad
• Nacre covers the bead

Non-beaded cultured pearl

• Mantle tissue alone, no bead
• Often a small hollow or organic center

Text version of this diagram

The diagram shows three pearls cut in half. The natural pearl has many thin rings of nacre that reach all the way to a tiny center, with a dashed ring around the outside for the pearl sac. The bead-cultured pearl has a large shell bead in the middle and only a few rings of nacre around it, inside the same kind of sac. The pearl grown from mantle tissue alone has rings of nacre around a small, irregular hollow at its center, again inside a sac.

How a natural pearl forms#

A natural pearl forms when cells from the outer layer of the mantle end up inside the mollusk’s body, where they keep making nacre. A predator, a parasite or a particle lodged in the tissue can move the cells there.¹

John Pfeiffer, curator of bivalves at the Smithsonian’s National Museum of Natural History, describes it as shell-making tissue in the wrong place. The misplaced tissue forms a pocket, the pearl sac, where a pearl can grow.⁴

Inside the sac, the cells coat whatever lies at the center. In pearl oysters they lay down nacre, and in other mollusks a non-nacreous form of calcium carbonate, so the pearl grows layer by layer.² GIA says natural pearls usually form around a microscopic irritant, and always without any human help.⁵

Natural pearls are rare. Hundreds of wild pearl oysters or mussels must be opened to find even one, and perfectly round natural pearls are extremely rare.¹

How a cultured pearl is grown#

A cultured pearl grows the same way as a natural one, except that people start it. For a saltwater pearl, a technician cuts a small piece of mantle tissue, the graft, from a donor oyster. The graft, about 3 × 3 mm, goes into the gonad of a host oyster together with a polished shell bead.¹ The tissue then grows into a sac around the bead and lays nacre onto it.⁵

The beads are usually cut and polished from the shell of a freshwater mussel, and they range from about 2 to 14 mm, depending on the oyster species.² In black-lipped pearl oysters, the sac covers the bead within 14 days, and the first nacre appears about 32 days after grafting.¹

After the operation, saltwater oysters generally stay in the water for 1 to 2 more years before harvest.¹ Akoya pearls are often ready sooner, and many are harvested within a year.³ Each Akoya or South Sea oyster grows one pearl at a time. After a harvest, a South Sea oyster can be given a new, larger bead, and one oyster can produce up to four pearls in all.¹

Farms try to harvest when the water is cold. Nacre then forms more slowly, so the pearl’s surface comes out smoother and more lustrous.¹

Pearls can also form on a farm by accident. When an oyster rejects its bead, or a scrap of tissue forms its own sac, it can grow a keshi, a cultured pearl with no bead.² Labs call these non-bead cultured pearls.⁶

How freshwater pearls grow without a bead#

Freshwater cultured pearls usually grow from mantle tissue alone. A freshwater pearl doesn’t need a bead to form.¹ Almost all freshwater pearls are grown this way.⁷

The grafts go into the mussel’s mantle, up to 20 in each of its two mantle lobes.¹ A mussel usually produces 30 to 40 pearls at a time.³ Freshwater culture usually takes 1 to 5 years, depending on the method and the species of mussel.¹

Some freshwater pearls are now grown with a shell bead, the same method used for Akoya. Pearls grown from tissue alone often have irregular shapes, while these bead-cultured freshwater pearls are often near-round to round.⁸

Why the layers make a pearl shine#

A pearl’s luster comes from light reflecting off its surface and off the concentric layers of nacre inside it.⁹ The same layered structure produces orient, a soft rainbow shimmer, and overtones, secondary colors over the pearl’s bodycolor.²

In a bead-cultured pearl, the nacre over the bead is what you see. Nacre thickness is closely tied to how long the pearl was grown, and it can affect the pearl’s color, luster and durability.² GIA checks nacre thickness to make sure a cultured pearl is durable as well as beautiful.⁹

How long each type of pearl takes to grow#

Growing times differ by type, because each mollusk and farming method is different. The table gives each type’s range from the Learn About Pearls dataset, and the page how long pearls take to grow explains the timings.

How labs tell natural pearls from cultured ones#

Labs tell natural, bead-cultured and non-bead cultured pearls apart mainly with X-ray imaging. A bead-cultured pearl usually shows a clear line where the nacre meets the shell bead, and labs can measure the nacre’s thickness from the image.¹¹

Pearls grown without a bead are harder to identify. They show organic-rich centers, voids or fine lines, and because some of these also occur in natural pearls, a few results are inconclusive.⁶