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Quick Questions gives you the low-down on how oysters turn a tiny bit of gunk into a lovely, valuable pearl.
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Sources:
http://www.livescience.com/32289-how-do-oysters-make-pearls.html

http://www.humantouchofchemistry.com/how-do-oysters-make-pearls.htm

http://www.scientificamerican.com/podcast/episode/as-the-pearl-turns-13-06-27/

http://www.popsci.com/science/article/2013-06/why-are-pearls-round

http://www.geo.utexas.edu/courses/347k/redesign/gem_notes/pearl/pearl_main.htm

http://nature.berkeley.edu/classes/eps2/wisc/Lect17.html
http://ressearch.files.wordpress.com/2014/03/abalone-nacre-1.jpg
Most of the shiny, blingy 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 like the splinter in your skin or dust in your eye.

So the oyster deals with 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 mainly 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 a base layer of protein glue, and then the layers start to stack up. These layers of nacre are what 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 turns 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 usually 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's been put there. That sounds pretty irritating.

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