I have a large sheet of Selenite (approx. 18"W X 30"L X 3"H) which was removed from a sandy hillside in Baja Mexico. Both sides are brown from the sand. I want to clean them to be able to use it inserted in a coffee table and shine a light through it from below. I tried a little water and a medium stiff brush, but it didn't work as well as I had hoped. Can I use bleach or mild soap solution? any ideas?