Hag stones are a charm of folk magic that are found predominantly in European lore, and they are simply a stone with a hole in it. They are used to keep away hags or witches that may come a-knockin' at night time.
If you were to hang one on your bed post you would prevent a hag from riding your chest, (giving you a nightmare in the process), and putting one in the stable would stop the hag from stealing your horses for a joyride and returning them in the morning exhausted, which was thought to occur often in older times. (Man, I hate it when they do that). Even normal stones or pebbles scattered around the entrance to a house has been thought to prevent a witch from entering.
Although hag stones are used to prevent encounters with beings such as witches, the stones are, in witch lore, considered to bring the finder good luck and fortune, and be a sign of favour from the goddess Diana.
This conflict -it will ward them off, or bring them delight at finding good fortune - should make you consider if you want this to be your primary line of defence against invading nasties. There are a great many charms to ward against such things, from the simple horseshoe
to complicated spells. Protection has never been this easy!!
-Please note that I am referring to hags and witches as separate things. They are different things, and many modern witches find the term to be very uncomplimentary (crazily enough).