Just as Covert_Death said, pretty much anything can scratch your glass.
While I'm very careful, always having a thick case to make sure the camera lens is in a distance from surface when putting it flat on a table, etc.. my son on the other hand, puts the phone in a pocket, with keys and coins. The camera lens has some super fine hairlines, which don't effect the photos. And the front screen look pretty ok, but the back cover, look like skidded across rough concrete.