Having done this myself using the old LOTR SBG Moria Starter Set stone pillars, I find the best way of achieving this Erebor stone effect is simply painting grey marble as you would normally, and once the effect is complete, using a mix of Biel-Tan Green and Nuln Oil to wash the entire surface. I used three very light coats of the wash allowing 20 minutes drying-time between coats. I also did one side of the columns at a time, so in total the final look took approximately 4 hours for the four Moria pillars (drying-time), and maybe half an hour to apply the twelve coats to each column. It's not a strenuous process and I found the final result was impressive to say the least.
Hope this helps.
