Tooth enamel and bird eggshells have a link beyond their neighboring spots on the color spectrum: both are made of calcium compounds. As with, limestone, pearls and seashells, eggshells are made of a composite of calcium carbonate. Tooth enamel is composed of calcium phosphate that is arranged in a tough, crystalline structure called carbonated calcium-deficient hydroxyapatite, or biological apatite.
The Effect of Acid
Eggshells and tooth enamel will soften and lose minerals in the presence of acid. Calcium carbonate reacts with acid to form water-soluble calcium ions and carbon dioxide. Loss of calcium makes an eggshell soft and weak. When acid is applied to tooth enamel, hydrogen ions combine with carbonate or phosphate ions in the hydroxyapatite matrix, which releases calcium ions. The demineralization of tooth enamel can create etching in the surface of the tooth.
The Effect of Flouride
When tooth enamel is exposed to fluoride, some of the hydroxy groups (OH-) in the apatite molecule are replaced, by ion exchange, with flouride ions. The resulting compound, fluorapatite is less soluble and more resistant to acid erosion. Similarly when chicken eggshells are soaked in fluoride solution, the shell is less likely to soften in the presence of acid, possibly because a similar exchange of flouride ions with carbonate ions creates a calcium fluoride compound on the surface of the eggshell.