Eye Stone, the operculum or calcareous mouthpiece of certain species of small univalve shells. The stony-like substance, one third of an inch or less in its largest dimensions, presents a form like that of a turtle, a convex surface upon a plane base; and being placed on a smooth plate in a weak acid, as lemon juice, the evolution of carbonic acid gas from the carbonate of lime of which it is composed lifts it up and causes the stone to move about as if alive. A similar effect resulting from chemical decomposition is sometimes observed in animal bodies; and loaves of bread, Humboldt remarks, have been observed to move in like manner in the oven, whence the ovens have been called enchanted. He found the little opercula, called piedras de los ojos, or eye stones, regarded as great mysteries by the inhabitants of the coast of Venezuela near Cu-mana. They collected them in great quantities on the beach at Cape Araya, and made use of them to extract dust or any foreign substance from the eye, a purpose for which they are still collected and exported, and are kept by druggists.

Being introduced under the lid of the eye, the stone moves about by the motion of the organ, and any little particles it comes in contact with adhere to it and are finally removed with it.