An adder stone is a type of stone, usually glassy, with a naturally occurring hole through it. Such stones, which usually consist of flint, 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 Germany they are called Hühnergötter ("chicken gods"). In Egypt they are called aggry or aggri. Various 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. There are other claims that an adder stone comes from the head of a serpent or is made by the sting of an adder. The more modern and perhaps easier to attain artefact would 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. In Pliny's Natural History According to Ancient Roman natural philosopher Pliny’s Natural History, book XIX, adder stone was held in high esteem amongst the Druids. Pliny described rituals the druids allegedly conducted to acquire the stone, and the magical properties they ascribed to it. He wrote: There is a sort of egg in great repute among the Gauls, of which the Greek writers have made no mention. A vast number of serpents are twisted together in summer, and coiled up in an artificial knot by their saliva and slime; and this is called "the serpent's egg". The druids say that it is tossed in the air with hissings and must be caught in a cloak before it touches the earth. The person who thus intercepts it, flies on horseback; for the serpents will pursue him until prevented by intervening water. This egg, though bound in gold will swim against the stream. And the magi are cunning to conceal their frauds, they give out that this egg must be obtained at a certain age of the moon. I have seen that egg as large and as round as a common sized apple, in a chequered cartilaginous cover, and worn by the Druids.