black diamond just has little shards of metal in there to expedite its aggressiveness. Dont be over concerned if you are not reusing it, the worry with the coal slag is when you have it in a cabinet and it keeps breaking down. The tractor supply stuff is made to just get blown out and not reused. I can tell you that stuff they sell is pretty fine considering coal slag options starting at 12 micron. That is what I start with since it is in a cabinet and it get to 20 after being used a while. People are much more worried nowadays from the silica in the play sand we let our kids play around in.