I made a huge mistake and cleaned white shell stone with copper sulfate crystals mixed with water to eliminate mold growth. The end result was that it stained the shell stone blue! I should have known to try a small area first, but no. I have tried 30% vinegar, 100% acetone, muriatic acid, and baking soda. I have pressure washed. Nothing. I am going to try oxalic acid and lastly hydrochloric acid. My very last choice will be to use a power sander.
I was wondering if anyone knows of a chemical agent that will counteract/remove the blue copper sulfate staining in porous stone.