Smooth the cut area of the stone by placing a cabinetmaker's rasp against it and running it over the stone with long sweeping strokes.
Wet the stone with a damp cloth and smooth any rasp marks and remaining chisel marks by applying a steady pressure to a small riffler against the stone.
Place the stone where you can run water on it, and sand it under the water with wet/dry silicon sandpaper. Starting with 150-grit paper, sand the stone, let it dry and then inspect it for any remaining chisel marks or scratches. If there are still marks, circle them with the colored pencil and resand with the same paper.
If no marks remain, continue the sanding process by doubling the grit of the sandpaper. For example: after you have removed all of the marks with the 150-grit sandpaper move on to wet sanding with the 300-grit sandpaper. Continue sanding, drying and inspecting in this manner, doubling up next with the 600-grit paper and, finally, the 1200-grit.