Reply To: Contaminated Lap?/Over Cutting Facets During Polishing › Forums › Beginner Questions › Contaminated Lap?/Over Cutting Facets During Polishing › Reply To: Contaminated Lap?/Over Cutting Facets During Polishing June 29, 2020 at 9:12 am #6671 Alan BalmerKeymaster @alanbalmer Don’t know why this mixture ends up going black but it does. Swarf is always black, in my experience – nothing to do with a particular lubricant. Probably because nanoparticles of stone in lubricant don’t reflect light.