I'm with Jim on this -- I've been singing this song since the late '50s, and it was always "a stane wi' a ring", as Jack Campin points out when he quoted the various Rebels Ceilidh Songbooks.
It just feels much tidier on the tongue, and easier on the ear.

And, to assure Leeneia and Robbie, the authors of the books mentioned ( the Ceilidh Songbooks and Norman Buchan's '101 Scottish Songs') were well aware of the actual appearance of the Stone of Destiny, so there was no carelessness in their transmission of John McEvoy's great song, and I'm absolutely confident that this is what he wrote.