In The Scot & the Sorceress, Nyssa uses a witching stone to see into the future. This is a stone with a hole in the middle, and can also be called an adder stone, a hag stone, or a serpent’s egg. These stones occur naturally and are probably the result of water eroding the stone, but they’ve been considered special (if not magical) for a long time.

Pliny, in his Natural History, made note of the popularity of such stones amongst the Druids. This is probably the oldest written reference to a witching stone. Here’s a quote:

“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. It is wonderfully extolled for gaining lawsuits, and access to kings. It is a badge which is worn with such ostentation, that I knew a Roman knight, a Vocontian, who was slain by the stupid emperor Claudius, merely because he wore it in his breast when a lawsuit was pending.”

Such stones are mentioned twice in the Mabinogion collection of Welsh prose tales: a magical stone is given to Peredur, which allows him to see and kill a fearsome but invisible beast called the Addanc, and Owain is given a stone by a maiden which allows him to become invisible and escape captivity.

In folklore, hagstones are believed to be protective – good things can pass through the hole to you, but bad things can’t. You can wear a hagstone on a cord for good luck, hang one from your bedpost to keep bad dreams away, hang it over a door or window to keep evil spirits away, or have one on your ship to keep storms away. They are used to cast spells by some witches, and evidently can break spells, too.

Of course, they also offer a view into other dimensions when you peer through the hole, like the realm of the Fae or the dead, which is how Nyssa uses her stone in The Scot & the Sorceress.