You can use a few small fake stones stacked, make them very dark or almost black, and one of the stones (with a lesser coverage) with a reflective shader. Then you can use a PF to further mask the reflection (and or the stones it sits on) in patches, like in the photo. And perhaps mask further by distance to emphasise only close-by reflections. Play with the refraction and amount of reflection, but make sure you turn off RT!