These look like fairly large rocks, but they're actually very small grains of sand. The ball bearings that you can see -- one at the top and another near the bottom -- in the photograph were placed there to provide a sense of scale; they each measure two millimeters across (0.08 inches in diameter); the entire picture is just one inch square. In the reflection of those bearings you can see the four white LEDs from the special camera that took this picture.
Shot on a sand dune near Christmas Lake, Oregon, this photograph was a test shot for NASA's Mars Hand Lens Imager (MAHLI) camera on Curiosity. "This image has a resolution of 15.4 microns per pixel, which is about twice as high as the camera resolution on Mars Rovers Spirit and Opportunity," NASA explained. "Geologists can examine an image like this for information about the composition of the sand. In this case, the largest white grains are pumice fragments and the dark black and gray grains are fragments of basalt. Nearly transparent, slightly yellow crystals are feldspars. The crystals and pumice were erupted by Mount Mazama in its terminal explosion about 7,700 years ago; the volcano is known today as Crater Lake."
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