Bill Tindall, E.Tn.
Visual observation is not useful for assessing depth of scratch if the scratch profile could be different. Sharp diamond makes a V (more or less) profile that becomes a light trap of incident light. Surfaces can look black. U profile scratches from less aggressive particle that is plowing and milling the surface will appear brighter. I would be more inclined to judge scratch depth from the microscope. I'm skeptical the second stone is doing anything useful.