Stone texture can be quite tricky to create and I have used several methods to create it in the past. I found one way that is better than others and is quite simple to do. I add layers of colour starting with the lightest, which is what you want your highlighted areas on the stone to be. In the video below, I used several greys, black and some green. I blend it out using odourless solvent, and then layer again. After another blend, I then take an electric eraser and go over the area using a random scribble pattern. I am not too worried about being exact, but just want to lighten areas. It adds a rough texture to the stone. You can add colour like green and yellow to make it look like the stone is covered in lichen as well.
Check out the video below as I show you exactly how I created this effect.