I , and my students, were working on an acid/base lab when I went on a "clean all the glassware" spree. Unfortunately I had a flask under my hood that I grabbed in a very "hurricane-ish" way and this is the result. The chemical was silver nitrate and it permanently stains your hands, and remains there until the cells replensish themselves. It was a short 3 weeks ago and only my super dark pinkie remains stained. The moral of the story? Don't save the silver nitrate in solution in your hood!