Also known as "summoning stones"
The majority of the Professor’s notes in the book, Umbral Leaves are beside entries that discuss “speaking stones,” or areas where the space between the Planes is thin and it becomes easier to commune with outsiders (demons and devils). Research suggests the more stones in an area, the more powerful the summoning. Areas with seven or more speaking stones are called “speaking rings.”
Here and there the Professor makes reference to a man he names N whose cabin in the Watcher’s Woods was the site of a series of grisly murders. The Professor wonders if there truly is a speaking ring in the Wood as N insisted, or if the man was simply mad.
While exploring the Wood, they were led to a speaking ring containing seven stones by a shadowy, hooded figure.