snakestones: Information from Answers.com

Traditions about ring-shaped ‘snakestones’ are Celtic. Most are Welsh, but there are Cornish references too, the first being in Richard Carew (1602: 21):

The country people retaine a conceite, that the snakes, by their breathing upon a hazel-wand, doe make a stone ring of blew colour, in which there appeareth the yellow figure of a snake, and that beasts which are stung, being given to drink of the water wherein this stone hath bene soaked, will there-through recover.

Such ‘rings’ were usually small prehistoric beads of striped glass, taken from ancient burials; however, Robert Hunt was told that snakestones were ‘about the size of a pigeon's egg’, and a friend of his had seen one which was a beautiful ball of coralline limestone, the coral being thought to be entangled young snakes (Hunt, 1865: 418). The oldest tradition, attested by Pliny in AD 77 and ascribed by him to Druids, was that in summer, at a certain phase of the moon, numerous snakes entwine and form a stony ‘egg’ from a sticky slime issuing from their mouths. He had seen one: ‘It was round, and about as large as a smallish apple; the shell was cartilaginous, and pocked like the arms of a polypus’ (Natural History, XXIX. xii).

In English folklore, fossil ammonites too are called snakestones. Legend claims they are coiled snakes decapitated and turned to stone by St Hilda, Abbess of Whitby, or alternatively by St Cuthbert; trade in ammonites flourished at Whitby (Yorkshire) and Keynsham (Gloucestershire).