Foam Aquascaping?

[Black Phantom]

Ok Here's what I've come up with so far. I'm going to build the main structure out of pink rigid foam. (no offgassing, stable, that sort of thing) Then a coat of marine epoxy to seal the foam.
I"m then going to try mix a batch of clear epoxy with black sand. I'll cover the structure with this mix to give me a rock look.
First off, where can I get black sand or fine rock in Calgary
Second, does anyone see a reason for this not to work. I don't want to put in all this work to have it fail for one reason of another.
Or let me know if you have another idea that might work.
Thanx