Hag Stones

Hag Stones, also known as Witch Stones, Fairy Stones, Holy Stones and Eye Stones, are stones with naturally formed holes running clean through them. These, all collected from the beach, protect my Gran's front door and back wall.

From: http://mcrocks.websitetoolbox.com/post?id=3041429
One of the most widespread magic devices to protect both man and beast was a pebble with a natural hole in it, also called ‘hagstone’, ‘witch-stone’, or (in the north-east) ‘adder-stones’. They were believed to repel witchcraft, and consequently any disease caused by spells or the evil eye; in particular, they prevented hag-riding. The earliest allusion is in a 15th-century charm against nightmares.Small ones could be carried in the pocket or hung up over the bed; larger ones were used in stables over Horses to prevent witches from riding the horses at night and bringing them back sweating and frothy. A variation, still known in the mid-20th century, was to hang the stone on the stable door; usually the doorkey or a bit of old chain would be attached to it, reinforcing its power with that of iron. Boatmen in Weymouth fastened them to the bows as charms to keep their boats safe. Small fossil sponges of the species Porosphaera are commonly found with natural holes in them; in Victorian times, necklaces of them were sold ‘for luck’ in Brighton (Sussex), and were much worn by women of fishing families.Some cultures believed that you could see Fairies or goblins by looking through the hole in the stone.