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