Just add a little black, mix but not until all the way blended (still marble/streak looking), the MMF will turn gray long before it turns black. Take a little MMF out of the batch and add more gray to the rest. Repeat as many times as you want. This will give you various shades of gray rocks for more realism. You might also add just a touch of brown to some of the MMF for some variance in the color. HTH.
Sorry, double post!
Thank you all for your replys!!!!