I started my first rock garden about two and a half millenia ago, with some grains of sand I received from Gautama Buddha. I nurtured them, adhering to a strict regimen of hard water. By the time I'd been reincarnated for the 64th time, my rock garden now has pebbles in it. It'll be some time before I achieve river stones. I knew someone who cheated and collected some river rocks in 321 AD, but that really messed with his karma: some farmer took them to use for a wall.
These are my own opinions based on wisdom earned through many wrong decisions. Your mileage may vary.