Elf rings, or Goblin rings, also known as sorcerer rings in France and as a witch's circle in Germany, were "scorch marks" left behind when a portal opened between the Visible and the Invisible.
The Twelfth Doctor explained to Bill that, in the time of the Fairy Finder, the locals of Huckensall believed elf rings were caused by the passage on elven feet, the Fair Folk cavorting with witches and warlocks. (PROSE: The Shining Man)