A Faery Portal
If you should encounter one of these in a tree or on the ground - CAUTION! Do not go nearer! Back away! This is a Faery Portal to their world. It can be in the hill or even lead to elsewhere!
Greenish Connemara Marble
The stony material that is referred to used as a key for unlocking a Faery Portal in the play, 'The Faery's Kiss'.
A Stone to See into the 'Otherworld'?
A Faery Stone: It has been said that a stone that has a naturally occuring hole in it (such as made by the action of water) may allow you to see Faeries.