1. Ninety-eight percent of an egg shell is calcite, which is calcium carbonate (CaCO3). Calcium is the major component of bone as well (in the form of calcium phosphate, or apatite). The calcium that goes into the shell comes both from the hen’s diet and from her bones. The average weight of a chicken egg shell is 7.26 grams. A truly remarkable hen can lay an egg a day. In a year, a hen can put into her eggshells over 25 times her own skeletal weight in calcium.