I think that raising 'pancake effect' would really help you here. You'll want some of the stones to bulge outwards from their bottom so they appear to be sitting on the ground, and a little under it, this will remove most of the sharp edges from the outside of the stones. Try pancake values of 1 to 2. You could raise 'tallness' to about 0.75, too, to accentuate their shape.
Your displacement's 'smallest scale' appear to be a little too small for the amount of displacement you've applied, too. Try raising 'smallest scale' or, lowering displacement by about half, keeping the scales the same..
Hopefully that's of some help.