bugs,
What does "true natural setup" really mean? Are you setting up a tank for enjoyment's sake or for the sake of being "true"? The fact of the matter is that there is absolutely nothing "true" or "natural" about any aquarium. No natural system for example uses flourescent lights, electric powered filters or electric heaters. The collection of plants in any natural biotope hardly ever resembles the manicured presentations that win contests, and neither do they contain the combination of species we normally assemble.
The term "nature aquarium" was never meant to signify anything other than a design concept which was departure from the traditional "Dutch" aquascapes (which were based on the principles of flower arrangements.) Nature aquarium designs are caricatures or impressions of a scene conceived in the mind of the designer. Ironically, these scenes are more often than not terrestial in origin - a shady glenn, a rocky outcropping, an open savanah , a mountain valley. It only takes one viewing of a natural history documentary on television to realize that most natural freshwater aquatic scenes are rather drab and dreary. So I don't think our objective ought to be to try to imitate nature, since this would be not only hopelessly impossible, but even if acheived would be hopelessly boring. Instead, think about using nature as an inspiration in your design to acheive the realization of the idea of a natural setting.
You should therefore get a gravel topping that
A) helps to grow plants well.
B) reminds you of , or implies a natural setting when implemented.
C) is pleasing to you.
That could turn out to be Amazonia, pea gravel or Corn Flakes or anything in between, depending on the limits of your imagination. Successful execution of a "natural" looking scene using whatever props or materials are available will demonstrate the difference between art and forensic taxidermy.
Cheers,