What is an egg shell made out of ? What makes it so hard ?

Question

What is an egg shell made out of ? What makes it so hard ?

Answer 1

Mostly calcium, a little bit of magnesium and phosphorus, and trace amounts of proteins and other things.

It's hard because of the calcium, which structurally makes up a good 98% of the eggshell, and the shape, round shapes are very good at distributing force around the structure.

Answer 2

It's made of calcium, like bones.  Oyster shells are the same thing.  When you have egg-laying chickens it is always a good idea to feed them crushed oyster shell to replenish their calcium levels.