Here is how I paint: pebbles with white veins
I use a reference picture, crop it to suit my paper or canvas, then draw it out.
I apply frisket to the white veins in the rocks. When dry and removed, this leaves stark white areas, which must be toned and adjusted until the veins appear to be part of the rock, instead of sitting on top. The veins themselves will have shadows where they go under the rock.