Apparently, Opportunity has been flipping rocks over on Mars. At least, that's the likely explanation for the donut shaped stone that suddenly appeared in front of the rover.
But, there's something odd - it has too much sulfur, magnesium and manganese - the composition is very wrong for a bit of Mars. I wonder if it isn't (i.e., is this a bit of an ancient meteor?).
Or maybe we just don't know as much about Mars as we thought.