What is the snail shell made of?

When a female snail lays eggs, she coats them with her "slime" and when the young snails hatch, they're still growing shells. Shells are necessary for snails to live and the "slime" helps harden them. If a snail shell cracks, substances like wax would not harm it and in the event that a snail shell breaks, it is reccamended to draw on it lightly with a crayon to hold it together. I'm not sure what the shell is exactly made up of, but I hope that this helps.