From Wikipedia, the free encyclopedia

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. In Britain they are also called hag stones, witch stones, serpent’s eggs, snake’s eggs, or glain neidyr in Wales and milpreve in Cornwall. 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, and of course recovery from snakebite. According to popular conception, a true adder stone will float in water.