Hag Stones are are stones that have a naturally occuring hole in them. They have been found by archaelolgists all over the world. And are believed to be a highly sacred magical stone.
The Celtic belief is that if you look thru a Hag Stone that you can see into the Fae or Fairy World. They would tie them around their neck when traveling through the forest looking for Fairies.
Sailors tied Hag Stones to their ships to ward off evil, such as bad weather, small catches and even wore them around their neck as it is believed that magic cannot work on running water and that is how some of the stones are created. They belived that if you looked through a Hag Stone into the water you could see the magical realm of Mermaids and Sirens.
Other beliefs are that they ward off evil, help achieve pregnancy and help ward off nightmares. Many a couple have worn Hag Stones through the ages and tied them to their bed when trying to have a baby.
They are also beleived to be bringer of high psychic powers and heightened intuition. And are said to have a high vibration.
The holes in the Hag Stones on my local beach are created by Piddocks or Angel Wings. Small bivalve animals use small teeth to excavate the rock and literally burrow into the stones. They have a fleshy foot that grips the stone and rotates it until they have a safe tunneled hole. It then filters food from the Seawater. I have heard it said that they have a bioluminescence nature and literally glow in the rock holes. I have found some stones with pieces of shells inside, but never a whole Angel Wing.