Not urgent!!! We moved to this house four years ago. The toilet bowl in the upstairs bathroom was seriously caked with yellow brown lime scale. Over the years, I have managed to get this off by attacking the material with weak household acids. There remains a dark grey to black, depending on the thickness, deposit over about a third of the bowl circumference organised as vertical stripes of various sizes, width and length. This is not Carbonate, it does not react with acid. Any idea what it is and ideally, how to remove it.