Pearls have excited admiration since ancient times for the richness of their shades and the perfection of their shape. A close look at these singular treasures shows that all of them, even the stones that are not perfectly round, are solids of revolution. Spanish and French scientists have studied how a pearl rotates inside its shell and have put forward a physical explanation of their own. A detailed study of the nacreous surface led them to conclude that the profile of a pearl is in essence a kind of ratchet, that is, a wheel that turns in one direction only, and that the axis of rotation of the stone coincides exactly with the axis of the ratchet. The rotation is slow. A pearl takes a whole month to make one turn.
The surface of a pearl is covered with microscopic teeth, which left the scientists with the question of whether the teeth are the cause of the rotation or whether, on the contrary, such formations appeared on the surface as a result of the pearl's movement. The answer they arrived at was more than expected: the two processes condition each other. As soon as a projection appears on the surface of the stone, molecules of the solvent settle on its wall and heat it, so that molecules of the dissolved substance fly off from it at a far greater speed than they arrived with. It is this excess momentum that forces the pearl to rotate. This is the curious way in which nature makes real treasures.