hag-stone

Grose also states, "that a stone with a hole in it, hung at the bed's head, will prevent the nightmare, and is therefore called a hag-stone."

Rookwood William Harrison Ainsworth 1843

Otherworld, it has a toad that spits out a hag-stone at my heroine's feet, which allows her to see what cannot be seen, and hear what cannot be heard; it has a gargoyle that comes to life, and a fairy creature that masquerades as a cat, and all sorts of strange and marvellous creatures from Scottish fairy tales.

Even in a little thing gillpolack 2010