Hag stones/Odin stones/witch stones/fairy stones…and many other names.
Stones with a natural hole in them are known for being a symbol of protection.
Wear them to protect your person. Place them in doorways and windows to protect your home. Hang them, or place them, near or on your bed to protect from nightmares.
If a stone breaks, it is said it has protected a life.
If one looks through the hole it is said to magnify and assist with clairvoyant sight.

So far I have found two (one in a forested area the other by the ocean) they are unmistakable when you find them, you will feel it.