so if its unique to the area then it wood have been noticed by neolithic man and revered because of this, hence the central position and the fact that the tomb was built around it and not incorporated as a structural support stone.

Or, it was bought into the area, possibly for the same reason.

Or, its just a fluke of stone texture.

Whats needed is a sample.
Being a protected monument chipping bits off is out of the question but someone like Rob may be able to get permission to get a core sample as was taken at Stonehenge leaving a plug that is unnoticable to most people.
(I haven't found anyone yet who can show me the plugs at Stonehenge.)

this is a job for the experts not any old sap,
PeteG