Unhewn stones: ‘whole’ or ‘peaceful’ stones, the adjective שלמות being of the same root as שלום, ‘peace’. The Altar, whose purpose is the forgiveness of sin, can only fulfill its mission when peace and brotherhood reign in Israel.

 Rabbi J.H. Hertz Commentary, p.862 (quoting Mekhilta of R. Yishmael, – a passage  in the name of R. Johanan ben Zakkai)