6789769446_f81b0aaef9_b

Winner: Calcite, Pupa Gilbert
The forming end of the tooth of the purple-spined sea urchin Arbacia punctulata, imaged with a scanning electron microscope and false-colored. One would never guess from their intricate and rounded shape, but these are single-crystals of calcite (CaCO3). They fill space, resist fracture and make the tooth hard enough to grind rock.