According to George Orwell, coal miners could develop characteristic tattoos owing to coal dust getting into wounds.[6] This can also occur with substances like gunpowder. Similarly, a traumatic tattoo occurs when a substance such as asphalt is rubbed into a wound as the result of some kind of accident or trauma. These are particularly difficult to remove as they tend to be spread across several layers of skin, and scarring or permanent discoloration is almost unavoidable depending on the location. An amalgam tattoo is when amalgam particles are implanted in to the soft tissues of the mouth, usually the gums, during dental filling placement or removal; another example of such accidental tattoos is the result of a deliberate or accidental stabbing with a pencil or pen, leaving graphite or ink beneath the skin.