Anything that's dry should be fine (no warranty issue). I tried baking soda a few years ago, but powder doesn't look "natural" to me. I didn't understand it fully, until I read The Physics of Blown Sand and Desert Dunes (1941). Turns out that if particles are small enough (~100 microns or smaller), they get "sticky" due to Van der Waals force. Though the angle of repose is maybe higher for powder, the way the dunes look just doesn't appeal to me. But I'm fine with people experimenting - though I'd advise against using glitter (I'm still finding it 3 years later :).