Eggshell n. : The shell or exterior covering of an egg. Also used figuratively for anything resembling an eggshell.

Eggshell n. : A smooth, white, marine, gastropod shell of the genus Ovulum, resembling an egg in form.

Example:
What is the chemical formula of an Eggshell?

Taxonomy
eggs, shell, eggshell, lay, the egg, chicken egg, layers, Eggshell skull rule, Bird eggs, reptiles, free encyclopedia, protein, uterus, extraembryonic membranes, birds, mineralized, Insect eggs, amphibian.

Defination from the web:
eggshell is the outer covering of a hard-shelled egg and of some forms of eggs with soft outer coats. The generalized eggshell structure, which varies widely among species, is a protein matrix lined with mineral crystals, usually of a calcium ...