Some individual Pearl beads are worth millions

Update:26-06-2018
Summary:

Some individual Pearl beads are worth millions—the jewe […]

Some individual Pearl beads are worth millions—the jewellery they're worked into even more. In 2000, the average annual value of West Australian pearls was around $220 million.
That's because pearls are made out of the same type of calcium material that bivalves make their shells out of.
"But not all pearls are beautiful, and not all pearls are valuable" .
Some bivalves (in this case, several species of oyster) coat their pearls with a hard material called nacre. This coating gives the pearl its recognisable iridescence and value in the market place and is the same material that lines the bivalve's inner shell.
Why exactly do little white nuggets from the sea cost so much? And how have humans hacked the biological process that makes them?
Pearls. Humans have prized them for so long that the very word itself has come to mean something of value.
But at any Asian pearl market worth its salt, you're able to find a string of pearls for just a few dollars.
So what makes one hard white lump from the sea that much more special than another? And how have humans managed to hack the pearl-making process?
What makes a pearl a gem?
"Biologically speaking, every mollusc that has a hinged shell (a bivalve) can make a pearl," Marie-Lise explains. That's right, even your chilli mussels were once capable of popping one out.