Hag stones or Adder Stones,  are created by water wearing down at the weakest point of a rock. 
They have been considered sacred by many belief systems. Some believed you could see through the guise of a fairy or witch if you looked through the hole. Some believed they were lucky, and were and still are made into amulets for luck or protection. 
They hold the power of the gracious water element, and the wonder of the seas and rivers. This makes them perfect tools and symbols to work with water deities. It can be a tool to embrace the flow of life, and connecting you to your own inner harmony, reminding you that there is an Ann and flow to life.