I get most of my rocks from Mama's Minerals, a fancy rock store here in Albuquerque (great fossils there as well as collecting tools), but just today I also got some at a new-age show ($1 each for some pretty stones about as big as a silver dollar but 1/2" thick or so). I look for jasper (a nice natural red), polished petrified wood, and various crystals (clear, pink, orange, purple). They all make great decorating materials, as well as being useful for holding down plants. I feel the rocks to make sure there are no areas that a fish zipping along might scrape against (thus I like the polished stones). If you find good stones out in the wild (i.e., in landscape or even in forests or deserts), you might want to beware any that are near sources of pollution (driveways, streets, mines), and of course check them with vinegar to see if they react and might leech minerals. The rocks I get at the rock stores I boil.