Most of the shiny, bling-y things that we use for jewelry

come from deep in the ground, mined from the earth's crust

and polished into some of the world's most expensive rocks.

But not your grandmother's pearls.

As you probably know, those shiny things grew inside the highly calcified bivalve that we know as the oyster.

So how did this happen?

And why?

Whether cultured by humans, or naturally formed in the wild,

Pearl formation is the result of a simple irritant.

Many species of bivalves, including mussels and clams, are capable of producing pearls when irritated,

but only a few can form the shiny coating that makes them so attractive to humans.

And oysters do it better than anyone else.

In the wild, the irritant is just a small particle that makes its way between the oyster's soft tissue

and its hard outer shell.

You often hear about pearls starting with a grain of sand,

but more often, it's just a random bit of gunk,

like a chunk of food that ended up in the wrong place.

But, no matter what it is, this foreign object can aggravate the oyster's soft tissues,

much as a splinter in your skin, or dust in your eye.

So the oyster deals with this irritant first by surrounding it with a thin layer of protective cells,

forming what's called a pearl sac.

These cells then secrete a combination of proteins

that form a kind of molecular glue around the offending bit of grit.

The sac then starts releasing layer after layer of material called nacre.

Also known as mother of pearl, nacre is mostly composed of a crystallized form

of calcium carbonate called aragonite.

Chemically speaking, it's the same compound as the oyster's shell,

but that kind of calcium carbonate, called calcite, is more durable and arguably less lovely.

Inside the pearl sac, the aragonite bonds with the base layer of protein glue,

and then the layers start to stack up.

These layers of nacre will give the pearl its iridescence.

But despite their smooth, glossy appearance, they actually have a slightly jagged texture.

Scientists think this allows the pearl to be rotated easily by the flowing water, which in turn allows

the coating to be distributed evenly.

And since the irritant itself was probably irregular in shape and shifted around while it was being coated,

most pearls aren't perfectly round.

The ones that are have usually been cultured by humans,

and those are made by implanting oysters with bits of tissue from other oysters,

and sometimes, spherical beads, to stimulate the formation of a pearl sac.

Often in the oyster's gonad, where the oyster can't dislodge whatever has been put there.

That sounds pretty irritating.

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Subtitles by Skylar Coland via Amara