These stains were caused by the constant dripping of water from a leaking gutter above, onto the stone surface below. These stains were by far, the worst I have ever removed...and will probably ever have to remove. They were so severe that there were literally layers to the stain. The first layer just flaked off. The second layer had to be removed with a razor. The third layer had bonded so well to the glass that you could barely feel it on the surface. Essentially, this was the worst case scenario as far as hard water staining is concerned. These 2 stains took 3 hours to remove, and the glass came out looking beautiful!
The stains were so severe that they left the surface of the glass pitted and damaged. We had to sand the surface (see below) to level it out again, and get it looking nice and smooth again.