If it were gouged out by an impact and then re-flattened somewhat by circulation, while a little dirt filled in the periphery, it'd come out looking just like this as well. As popular as these are, and as closely as these have been studied, I should think that a die chip visible to the naked eye in the middle of a field would likely have been recorded previously. This is no guarantee that it isn't a die chip, but to my mind the possibility is less than that of a worn postmint hit.