Your stone reminds me of similar stones found on local beaches here in Norfolk, UK:

these are rounded pebbles with natural, often centrally placed holes. According to mythology they were considered powerful magic and to ward off evil spirits. In European beliefs, the hag stone keeps the 'evil hag' spirit away and prevents it from stealing horses and children. It was also used as a talisman to ward off the evil eye. Some people kept a hag stone in their bedrooms to prevent the succubus-hag from riding on people's chests during nightmares. In Italian witchcraft, the holed stone is associated with fairies, and often referred to as 'the holy stone'.

In Norfolk, UK, it was thought that hags or witches would take horses from stables and ride them throughout the night until they were tired out. A tired horse was, of course, not much use for working the fields with. In order to prevent witches from "borrowing" their horses, farmers would tie a flint pebble with a hole in it - a hag stone - to their stable doors.

Image below depicts locally found stones, one is a so-called witch stone, or witch bead, if you may; the stone at left is similar to yours in that it too has a central circular depression?

Cheers,
Evelyn