  Pearls, the most flawless and highly prized of them, are perhaps the most
perfectly spherical macroscopic bodies in the biological world. How are they so
round? Why are other pearls solids of revolution (off-round, drop, ringed), and
yet others have no symmetry (baroque)? We find that with a spherical pearl the
growth fronts of nacre are spirals and target patterns distributed across its
surface, and this is true for a baroque pearl, too, but that in pearls with
rotational symmetry spirals and target patterns are found only in the vicinity
of the poles; elsewhere the growth fronts are arrayed in ratchet fashion around
the equator. We demonstrate that pearl rotation is a self-organized phenomenon
caused and sustained by physical forces from the growth fronts, and that
rotating pearls are a - perhaps unique - example of a natural ratchet.
