How Do Oysters Make Pearls?

The most popular origin story for pearls goes something like this: A grain of sand gets stuck in an oyster and eventually becomes a pearl. While this isn’t wrong, it’s definitely an oversimplification; and it’s also not true for every one of these gems.

For one thing, the ability to produce pearls isn’t specific to oysters. In fact, according to Smithsonian, all mollusks—from mussels and clams to marine snails and scallops—technically have the capacity to do so. And the process doesn’t always start with a grain of sand: It’s often an errant bit of food or some other irritant that ends up stuck in the creature’s tissue. As a means of protection, it’ll begin encasing that irritant in layers of the same materials its shell comprises. It can take as long as five years for a good-sized pearl to emerge.

How Mother of Pearl Comes In

Not every mollusk can produce the iridescent, lustrous gems you picture when you think of pearls. But oysters can, and that’s because their pearls are built from nacre—aptly known as mother of pearl. Nacre is created by alternating layers of aragonite (a form of calcium carbonate) and a protein called conchiolin. Though some people refer to other, non-nacreous pearls simply as “calcareous concretions,” they can be just as stunning as their nacreous counterparts. A sea snail called the queen conch, for example, produces them in varying hues of pastel pink.

These days, the pearl industry doesn’t leave much of the process to chance. Instead of cracking open oyster after oyster in the hopes that an irritant landed inside years earlier, pearl farmers actually slip an irritant—often a tiny shell bead—inside the oysters, leave them alone for some years, and then harvest the pearls.

[h/t Smithsonian]