Tahitian Pearls What Causes The Circular Grooves On The Surface Of Many Types Of Pearls, Especially Tahitian?

What causes the circular grooves on the surface of many types of pearls, especially Tahitian? - tahitian pearls

The rings are caused by the rotation of the nucleus in the bag of pearls.
"A chain through the introduction of cultured pearls is a round pearl grain and produces a small piece of mantle tissue donor. The acts of the donor tissue as a catalyst and a bag of pearls around the pearl. Pearl is the recipient of the Perlschalen and finally, the Pearl is . But the grain, and the Pearl, "as it develops, often" spin "in his pocket. This creates concentric grooves known around the pearl as circles.

PS
Arena does not create a natural pearl is an urban legend!