Calcium Carbonate

• About 95 percent of an egg shell is calcium carbonate, which has a chemical formula of CaCO3. Calcium carbonate, a crystal structure, provides a strong bond, while allowing space for other substances to exist, or for air and moisture to pass through the shell.
  
  

Other Substances

• Egg shells also contain calcium phosphate, magnesium carbonate, sodium, potassium, zinc, manganese, iron, copper and proteins. Such substances help make the egg shell stronger.
  
  

Fun Fact

• Although egg shells look solid, they contain an average of 9,000 pores and can contain up to 17,000 pores. As eggs age, carbon dioxide leaves the egg, which is what makes boiled, older eggs easier to peel than fresh eggs.