You have the basic idea yes
Paint up the stone with a deep grey colour, then get a lighter grey and start drybrushing.
Get yourself a nice bristly brush that you do not mind wrecking a bit, then add a small amount of paint to it, and wipe it repeatedly on some tissue untill most of the paint is gone.
Now simply drag the brush across the stone, making sure to cover it evenly with the barely paint wet brush, I find circular motions are good, but whatever you find to be most comfortable is fine.
Once you are happy with the result, let it dry, then wash all the crevices down with badab black, and some brown or blue ink added in.
Once dry, then try another slightly paler dry brush over it again.
You can see the kind of result you get on my High elf dragon in my gallery, as long as you do not rush it, it is lovely.
http://www.dakkadakka.com/gallery/22845-High%20elf%20dragon.html