The trick is to blow them up just a tad by an offset variable, either from a surface shader or a PF (so you won't see the hard edge on zero), and have the displacement add its positive and negative displacement on that slightly blown-up stone. Negative decreasing the blown up 'bubble' again, positive adding to it. But then you easily get stone sides crossing eachother, or the negative displacement is more than the offset and you see the hard edge again, so it's a careful balance.