Blueberries on Mars

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After landing in a small crater, Opportunity found many tiny sphere-shaped nodules, dubbed “blueberries,” that were rich in hematite and were probably formed by water below the surface. The blueberries average about 0.3 centimeters (1/8 inch) in diameter.
Mars Exploration Rover Opportunity,
Microscopic Imager NASA/JPL/Cornell image