Its gsp.... Tear it carefully and superglue it to the wall... I started with a nickel size non bright and nickel sized super bright gsp and just glued it on the back glass and overflow box
I just put my gsp rock next to the glass wall and it a matter of months it covered 50% of the back glass. Not sure if it has to do with different strains of GSP but you are welcome to a large chunk if you are ever in SD.
Alan, I did the same as Mike, just placed a piece of it up against the glass and eventually grows onto it and spreads out. I can also frag you off a nice chunk for free and I think I'm a little closer than Mike