Yes, Marion is perfectly correct. The Tor consists of layers of clay and Jurrasic limestone with a hard sandstone cap. This sandstone is Midford sandstone and contains within it much harder egg-shaped boulders known locally as "Tor Burrs". They vary from a few inches to a few feet across. This partricular egg-stone was found during excavations of the Abbey and all sorts of speculative pagan religious stuff became attached to it. At some point, a notch was cut into it and it was almost certainly used as a cross base. Sad to hear that it is now covered with swastikas and other symbols - if only today's "pagans" would take some trouble to find out exactly what they are venerating - or defacing! Its just a natural boulder, but as they tend to emerge from the hillside as soil is eroded, they have been seen as magical. I am not certain, but I believe that this stone, or another very similar, is also known as a "growing stone". This is very similar folk lore to the slowly emerging puddingstones of Essex and Hertfordshire.