Adder Stones (sometimes also called hag stones, witch stones, serpent’s eggs, snake’s eggs, Glain Neidr, Milpreve, adderstanes, Gloine nan Druid, aggry, or aggri) are stones, usually smooth or glassy, with a naturally occurring hole bored straight through it, typically from water erosion or natural damage.
That said, there are no shortage of stories claiming these stones have a more unnatural origin. One claims that they are formed from the hardened saliva of a great number of serpents, massed together, and the holes are from their tongues. Another claims the stones are actually the heads of snakes.
Adder stones feature prominently in Welsh mythology and Druidic culture. They were believed to have magical powers and thought to protect people from a gamut of problems, ranging from eye diseases, evil charms, nightmares, whooping cough, and snake bites. It was also believed that if you looked through the hole in an adder stone, you could see through the disguise of a fairy or witch.