I went hiking in pikeville and came back with a TON of shale rock. Its awesome because I can break off what I need and to fit it how I need.
I recommend with this as well as any type of shell or rock to thoroughly clean and bleach in your tub for an hour or two, then rinse in scolding hot water, repeat process, then rinse in cold cold water, let dry. Then heat in your over at 350 degrees for 30min to an hour, rinse in cold water again. Then put in your aquarium.
Be careful about what types of rocks and things you use because some (like shells and drift wood) will adjust your PH a bit.
I also have a bunch of multicolor flat river stones I picked up from hiking/swimming at the blue hole on Montlake in Soddy Daisy. There are a lot of awesome rocks there especially nearby the waters edge. The water is so clear in most areas as well so you can look and take your pick before swimming down to grab it.
I often take my dogs with their backpacks and put a few really nice ones in each ones pack. They love to swim and work, so they always have a blast.