The Encyclopedia of Superstitions
The name 'adder stone' is commonly applied either to a small stone with a natural perforation in it, or to a certain type of coloured, glass-like pebble which is fairly uncommon and consequently has all the virtues of the strange and unusual.
Prehistoric spindle-whorls turned up by the plough are also frequently so called by those who do not recognize them for what they really are.
Such stones were formerly prized as charms against evil and were used in cures for various human and animal diseases. According to tradition, they were generated by snakes who, at certain times of the year, congregated in large numbers, coiled themselves into a living ball and, while so united, emitted saliva which hardened into a stone. Because of their serpentine origin, they were widely used to heal adder-bites. Many stories are told of their efficacy for this purpose. One such tale, related by William Henderson in Notes on the Folk-Lore of the Northern Counties (1879) records how a nineteenth-century Scottish labourer near Pitlochry was badly bitten by an adder. 'Severe pains came on,’ said Henderson's informant, 'and a terrible swelling, which grew worse and worse, till a wise woman was summoned with her adder's stone. On her rubbing the place with the stone, the swelling began to subside.' They also cured ague and whooping-cough, kept away nightmares if hung over the bed, and when carried about in the pocket, prevented eye-troubles.
Although their healing properties are not now much regarded, their use as luck-bringers and protective charms is not yet entirely extinct, and in some parts of Great Britain and Ireland such stones, especially those accidentally found, are still preserved and handed down as amulets.
Text from The Encyclopedia of Superstitions, by E. and M.A. Radford, edited and revised by Christina Hole (Helicon Publishing, 1995)