Thank you all for your comments.
To Archonforest : I am using several differents layers of stones of different size here along with merge shaders. I am using negative perlin billow noise displacement with quite high contrast colors.
The thing which requires more adjustment works is the noise/color/displacement scale of each layer and depending on the scale of the whole scene, you have to scale them down or scale them up until you find a good result.
Same thing for the density variation scale : here I am using quite high distance with density varying from 1 to 5 depending on the layer (5 for the biggest stones).
I hope this can be of some help for you. Perhaps will I try to do some kind of kick tutorial if I have time one day.
David.