Symbolizing 'A Thing
That Is Not A Thing'
Attempting to summarize atheism in an icon that would be universally and immediately recognized is an exercise in futility. Atheism itself is "a thing that is not a thing." On top of that, is the subjectivity of the viewers whom we would be trying to impress. Not realizing that it was Ash Wednesday and that he had just returned from a Roman Catholic mass, I once pointed out to a coworker that he had a smudge on his forehead.
Those things aside, if you want to wear or display a symbol which may spark discussion toward atheism, consider these:
1. A question mark (because we tend to question things rather than just accepting by faith)
2. A small mirror (which reflects what we can see, this is what we tend live by realistically and metaphorically)
3. A lapel button (like the kind with sayings on them) with no writing, just solid black. (It's nothing)
One might even consider some combination of these things.