I've seen people use sheets of tinted window material. It's like plastic, and can br removed easily if you get tired of it. There's all sorts of colors. However, they don't seem too natural.
I'm getting ready for a bigger tank, and my plans are to find a nice color marble or granite cut in the raw (that's the stuff they use for floors). And since I'm looking for the one with holes and crevices, I'll surely pay peanuts for it. I was thinking along the lines of a nice Carrara, which is a natural lime-cream color, so as not to compete with the fish colors.
It would have to be placed on the outside though, because of the weight and porousness.
B)