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This image from NASA's Curiosity rover shows the great diversity of grains found on the surface of a Martian rock. The rock, called "Gillespie Lake," was imaged by Curiosity's Mars Hand Lens Imager on Dec. 19, 2012, the 132nd sol, or Martian day, of operations.
The grains highlighted in the annotated version of this image are about 1 to 2 millimeters (0.04 to 0.08 inches) long and the full frame is about 3 to 5 centimeters (1 to 2 inches) across. Some grains look similar to clear crystal in appearance while others are darker and duller in appearance.