In some folk magic practices, these stones are believed to ward off or break hexes, sickness, and nightmares. I think most of that came from an old tale that tells that curses and bad luck are caused by spectral hags.
It is said that when you look through the stone itself, you can see other realms. Some traditions state that you can see the Fae, other dimensions, and even nature spirits.
You can hold it up and look at the full moon through them and speak your desires or what you want to get rid of.