How are eggs formed inside the chicken?
-- Joan Peters

The egg actually grows the shell around itself! It does this using processes that are also seen in bones and seashells. Around the egg is a membrane, and evenly spaced on the membrane are points where columns of calcite (which is a form of calcium carbonate) form. These columns stack together side by side to form the shell. The nucleation points are defined protein nodules called mammillary protrusions, and the mineral is first deposited as particles of aragonite with random orientation of the crystal planes. On top of these aragonite piles, columns of oriented crystalline calcite begin to grow upward. So, the calcite is like floating in solution around the shell, and it deposits on the shell like a forming crystal.