Spanish researchers say pearls are the most perfectly spherical large objects in nature because they turn inside oyster tissue in response to movement.

Researchers at the University of Granada note flawless and highly prized pearls have perhaps the most perfectly spherical shape among all the objects in nature that are visible without a microscope.

Writing in a journal of the American Chemical society, they say the spherical shape results from a developing pearl's saw-toothed, or ratchet-like, surface, a texture that creates forces that cause the pearl to turn inside the oyster's tissues in response to movements in the environment.

The ongoing movement results in a spherical build-up of nacre (mother of pearl) deposited around a grain of sand or other foreign object in shellfish.

The rotating pearls may be unique example of a natural ratchet, the scientists said.