Call of the Dark Rocks 
This false-color image (below) taken by the Mars Exploration Rover Spirit shows a group of darker rocks dubbed “Toltecs,” lying to the southeast of the rover’s current position. The rocks are believed to be basaltic, or volcanic, in composition because their color and spectral properties resemble those of basaltic rocks studied so far at Gusev Crater. Scientists hope to use these presumably unaltered rocks as a geologic standard for comparison to altered rocks in the area
Image Credit: NASA/JPL 