Crack eggs and save shells. Wash shells out immediately after cracking eggs. Bake shells at 350 degrees F for about 10 minutes to remove the mineral oil coating and make them brittle enough to grind.
Grind to a fine powder in a food processor or blender. Make sure there are no sharp, gritty pieces.
One whole eggshell makes about a teaspoon of powder, which yields about 1,800 milligrams of calcium.