Wow! I like the river rock ones! I may have to try that! Be aware though that the silicone that shows will become more visible underwater (I have no idea why, but it is very annoying). Also, depending on your water, it may turn white after a while. This happened to me when I glued a bunch of lava rock together for retaining walls. Eventually, the white was covered in algea and it is now not really visible at all. The one way that I found to prevent silicone from showing up was to sprinkle sand on the wet silicone and really pack it on.
Another thought...for the slate trapezoid in your pictures...make sure that one is far enough away from your glass that if it collapses it wont hit the glass on it's way down! I had a slate "table" sort of arrangement in my tank once, and the silicone gave out...it just collapased downwards, so no harm done. But if it had hit the glass...