Hi Paul,
there are a few ways of achieving that. None of them is extremely intuitive, though.
You could use the fake stone shader as the blend shader for a distirbution shader. Select invert blendshader inside the distribution shader dialog, then add a color adjust shader below the distribution shader. Increase the black point in the color adjust shader (e.g. to 0.5). Then use the color adjust shader as your new distribution shader in the population.
Regards,
Frank