For “only” round voronoi you might look into something like this:
But in your reference there are some stones are more round even like “blown up” (red) a little and so the other stones have to “shrink”. And also some of those seems the “merged together” into some kind of inbetween stones (green).
…so the question would be how to do this :
- in somekind of 3D texture
- with geometry
and for both how to grow some stones and shrink other and when to merge… 
General idea:
Select some “random” stones and do some spherical resize with a bigger radius than the stone to shring the neighbours ( the gap might grow bigger ). Second: when some smaller stones adjacence to some other smaller stone… merge them randomly (otherwise maybe too much get merged…)…