I had some aquarium safe silicone from when I fixed my little six gallon and thought "hey, let's make some crap." I picked out some suitable rocks we had lying around the back yard, scrubbed them in hot water, soaked them, and scrubbed again. Once they dried I sure had a time figuring out how I would assemble them. Took me a while to get a system going, but third time's the charm. And it actually came out looking pretty good. The photos don't do it justice.
Here's the arch made from smooth river rocks. I love how it looks. Stylish, don't you think? :)
Next on my project list are some caves made from piping covered with gravel. I'm also going to make a few from terra cotta pots, as well as a castle/cave/tunnel system.