This false-color image shows the rock "Chocolate Hills," perched on the rim of the 10-meter (33-foot) wide "Concepcion" crater. Patches of unusual dark material can be seen on top of this rock and on several others in the scene.
Opportunity Rover: Chocolate Hills Rock
Opportunity Rover: Chocolate Hills Rock
http://marsrovers.nasa.gov/gallery/pres ... 0211b.html