i got black gravel in my 10 gal, looks nice so far, and i got some stones from outside in my 2 other vases, they are pretty though, and of course is scrubbed and washed them in hot water before i put them in :)
Depends on what theme I want. Right now I have bamboo stalks and fine gravel for the bottom with a few jewel stones sprinkled on that. I also have a few black river stones that look "Asian". Goes great with the bamboo.