We know how oysters make pearls, but how can they be perfectly spherical? A new theory says their remarkable structure allows them to naturally rotate in the shell.
One of the most beautiful and impressive of the many and varied
defence mechanisms that occur in nature is the creation of pearls –
generated when foreign particles such as sand grains or parasites get
inside the soft bodies of oysters, clams or mussels.
When defences
are breached, these organisms respond by coating invaders in nacre, a
hard iridescent material also known as mother of pearl. The results come
in many colours, white, grey, black, red, blue, green or yellow, and
their attractiveness has led to traditions of pearl-diving that are
thousands of years old. Today, pearls are harvested in oyster farms in
the Indian Ocean, East Asia and all across the Pacific, where production
is stimulated artificially by inserting round beads into the molluscs
to serve as a seed.
Yet in spite of their history and commercial
value, we still don’t fully understand how pearls form. We know they
occur in a variety of forms, such as elongated and ovoid, or the
teardrop shape that works well for earrings. Some, called baroque
pearls, are irregular, like blobs of solder pinched off at one end. The
most highly prized specimens are perfectly spherical.
For the rest of the story: http://www.bbc.com/future/story/20130623-how-pearls-get-their-round-shape
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