Sorry, I keep editing this post. Hopefully that does not cause any problems with anyone. Let me know if it does, please.
Here are a few of the rocks that I am using. The stones look darker when they are wet.
To come up with a good initial design, I laid the plastic container down flat and just added enough stones to cover the bottom completely. Then I carefully stood the plastic container nearly on end and let the stones pile up, having the back of the container to keep them from collapsing completely. It gave a nice natural look, and helped me figure out how to assemble the rock-work.
With that image in mind, I dumped the stones out and began assembling them in the corner of rectangular plastic containers as a form, since am planning to put the completed rockwork in the corner of the tank. Securing the stones together using silicone is necessary with these smooth flat stones, or they would just slide off of each other and create a pile of rocks with no hiding spots big enough for any fish to hide in.
I'm building the rockwork by degrees as tiers. Below are two different tiers that I will try to stack up later in the project...hopefully. In the second pic you can see a rough cave taking shape. I have some added temporary stones in there to hold the pile in place until it dries, but you get the idea.
That's about as high I could stack them and keep them in place while the silicone was wet. I'll have to let the silicone dry overnight before I continue.
My next step is to remove the rockwork from the containers and add more silicone to the back unseen seams to reinforce it, if needed. Later, I will try to assemble the tiers one on another and fill in a few extra rocks at the seams and corners to round it up a bit and give it a more natural look. I'm also planning to add discreet silicone "bumpers" in the back to keep it from banging into the tank glass.
Lessons Learned: The aquarium sealant in a tube is rather difficult to work with. I wish I would have cut a smaller opening in the tube end so I could be more precise in the application. I had planned on using the silicone such that it was not visible at all, but this is proving to be challenging with the uncooperative, messy stuff. I will have to try to cut out/scrape off any excess with an X-Acto knife when it's finished. At this point it looks like I may need a second tube to complete the project. The good news is that it adheres to the clean river rock just fine. I also advise plenty of ventilation when using silicone. There were a lot of fumes, and I was a little woozy for a while afterward.
My stone inventory is about 1/3 gone already. I may need to forage out in the landscape for another load to clean and use. We will see.