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> For technique (more in line with your question), I know the irregular
> rock faces on many houses start with side board, which they nail
> chicken-wire on top of, then build a cement rock wall in front of
> that with the cement grabbing hold of the wire for additional
> support. I'm going to look into tile supply houses and see about
> doing a similar thing with rock, grout, and tile adhesive (I don't
> want to go with the plastic back-drop either). While my sides will
> probably be glass (greater viewing area), the back might be that
> "concrete wall-board" used for tiled areas (totally impervious to
> moisture, I'm told). 
> - --charley cbay at jeppesen_com, cbay at verinet_com 

Concrete wall-board is indeed impervious to water, but not necessarily
waterproof. I have used it as a base for tile in a shower and recall that the
sheets were perforated with numerous tiny voids, much like concrete block. I
would not be suprised if it leaks in the application you suggest. A little
experimentation might prevent a lot of grief.