I collected a bunch of river rocks from a riverbed in Virginia. They look to be some sort of metamorphic schist that are predominantly greyish brown with some flecks of mica. Quite nice looking IMO.
Here's the driftwood supported on a "tripod" of rocks. I ziptied it down to down the rocks to keep it stable and stop it from floating.
More rocks added
I'm using a simple sand substrate. Tried to find pool filter sand but none of the store near me had any so I ended up going with paver sand. It took a lot of washing to get it clean but in the end it looks quite nice. It's a nice tan color and a mixture of grain sizes. I also mixed in some light brown, dark brown and black sand to get a more natural looking blend.
Let there be light!