Good thought, it could be, easy to check out if they are they are the same type of stone. Mind you it's a wee little pebble compared to the altar stone. Our ancestors certainly had a penchant for special rocks.
My friend Peter, the paleoartist suggested the Pleistocene glaciation could have moved it. Stones moved like this have a great name, glacial erratics. Being an ignoramus about geology I had too check if stones this big can be moved by glaciation - they can. The various waves of Pleistocene glaciation did cover Scotland and Britain, so it's a plausible theory.
