I stumbled upon this blog post about hag stones. Interesting post so I thought I would share:
What is a Hag Stone?
A Hag Stone is simply a stone with a natural hole through it. In folk magic systems, these are often believed to ward off the dead, curses, witches, sickness, and nightmares. They are also used as windows or doorways to see 'otherworlds', invisible spirits, or how a being 'really' looks beyond their glamour.
Read the rest here: http://cauldroncraftoddities.blogspot.co.uk/2012/10/hag-stone-lore.html?m=1