It's some sort of white, very porous sand. It has a volcanic origin. It's nice to walk on when on flat ground, but when you encounter it on a steep slope it's horrible as it makes you slide back down all the time.
...a lot. I know the Sajama desert (Bolivia), i think is a similar landscape.
I have only been in the outskirts of the Sajama area, but from what I saw, it's similar to the Puna.
I like both places a lot.