I have my own chickens, so I use a lot of eggshells. Here is a little clip that I found, eggshells are 95% calcium carbonate but they have a little extra too.
About 95% of the dry eggshell is calcium carbonate weighing 5.5 grams. The average eggshell contains about .3% phosphorus and .3% magnesium and traces of sodium, potassium, zinc, manganese, iron and copper. If the calcium from the shellis removed, the organic matrix material is left behind.