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Hag Stone or Adder Stone as defined by Wikipedia:
An adder stone is a type of stone, usually glassy, with a naturally occurring hole through it. Such stones have been discovered by archaeologists in both Britain and Egypt. Commonly, they are found in Northern Germany at the coasts of the North and Baltic Seas.
In Britain they are also called hag stones, witch stones, serpent's eggs, snake's eggs, or Glain Neidr in Wales, milpreve in Cornwall, adderstanes in the south of Scotland and Gloine nan Druidh ("Druids' glass" in Scottish Gaelic) in the north. In Egypt they are called aggry or aggri.
Adder stones were believed to have magical powers such as protection against eye diseases or evil charms, preventing nightmares, curing whooping cough, the ability to see through fairy or witch disguises and traps if looked at through the middle of the stone, and of course recovery from snakebite. According to popular conception, a true adder stone will float in water.
Three traditions exist as to the origins of adder stones. One holds that the stones are the hardened saliva of large numbers of serpents massing together, the perforations being caused by their tongues. The second claims that an adder stone comes from the head of a serpent or is made by the sting of an adder. The third is more modern (and much easier to attain). It details that the stone can be any rock with a hole bored through the middle by water. Human intervention (i.e., direction of water or placement of the stone) is not allowed.