The socalled Altarstein (altar stone) made from Diorite, length about 3m with saw cuts made with a blade at maximum thickness of 2mm. In my opinion, all the work on the Felsberg have been done at a time when the granite was "new" and not completely hardened yet, in a state like wet clay. I know, that this contradicts the classic theory about granite formation, but I don't see any other reasonable logical explanation so far.
There is a new well designed and informative visitor centre at the foot of the Felsberg: http://www.felsenmeer-informationszentrum.de/
Location: Felsberg, Odenwald, Germany.