Clay? Kaolin? Ice chips? They are eaten in some cultures, and other folks may eat them too, especially if vitamin deficient. But of course they have no calories, which makes them rather useless as food. It's inescapable - Everything living is somewhere on a food chain, where depends on what part of life (or death) you happen to be in. Even crude oil/oil shale, those long dead oddly decomposed plants can be digested by a few types of bacteria.