Speaking for the various Witches' Stones in Perthshire:
The Witches' Stone at Monzie is supposedly named after Kate McNieven, who was supposedly one of the last witches to be burned in Scotland. She was burned in front of the hill that you can see in the background of one of my photos (the one with Scotty and Martin looking at the stone). Both Scotty and myself have left a bit about her in the folklore section: http://www.themodernantiquarian.com/browse.php?site_id=3267