The story that pearls start around a grain of sand is a myth!

Pearls are a product of a bivalve’s immune system—a response to something invading its shell and irritating its body.

The oyster will try to blow it out, like we do when a long strand of hair drops in front of our eyes. If that doesn’t work, the oyster will start coating it with nacre. That’s where a natural pearl is born, in between the shell of the animal and the part that’s called the mantle.

The mantle, the outer layer of the mollusc’s flesh, coats whatever is irritating it with calcium carbonate and nacre. Lighter and stronger than concrete, this pearl is the oyster’s attempt at protecting itself.

Old wives will tell you that it’s normally a grain of sand that begins this irritating, pearl-producing process. But we now know that’s not true.

If you were to cut through the middle of a natural pearl, you’d find organic matter there if it doesn't dissolve as time goes by!

Because  when an organic irritant—such as a tiny parasite, a worm fragment, or organic debris—gets trapped in the Mollusk's tissue, a pearl is formed as a response to the irritant! If you cut a natural pearl open, you will find concentric layers of nacre surrounding a microscopic organic core or a tiny void where the organic matter decayed.

What about cultured pearls?

Most pearls on the market these days are human-made. Technicians implant a smooth, round bead made from a freshwater mussel shell (along with a small piece of mantle tissue) into the Mollusk.

After implanting, the oysters are placed in pockets of mesh panels that are suspended from lines of rope in the sea. They’re left for about 18 months to grow their precious load.  Not all of them will produce perfect pearls. Some pearls will come out in funny shapes or with pocked and dull nacre. 

If you cut a cultured pearl open, you will clearly see that solid shell bead core surrounded by layers of secreted nacre. 

People report that they never find sand grains inside the pearls. I never found them too!

There is  no documentation of someone finding a grain of sand in the centre of a natural pearl.