Opercula are formed in a variety of shapes. The external outline matches the shape of the current whorl of whatever snail forms this "secondary shell" to act as a protective trap door.

Circular opercula derive from sometimes-large marine snails with round openings, and do tend to be domed and sometimes colorful—such as the cat's eye operculum from a turban snail. Viewing the flat underside (that connects to the "foot"), an operculum usually has a spiral structure, as do the primary shells. (Second and right photos here.) They do not have parallel layers, such that carving would present an "eye." But the opercula of the turban snail can be quite colorful and present an eye-like appearance (seen in the attached images).

Opercula are sometimes cut to make their shapes regular, and to fit into bezzels. Recently white opercula set in silver have been plentiful in Indian jewelry, and are called "Shiva eyes."

JDA.