Biomineral crystals found in a sea urchin tooth. Geologic or synthetic mineral crystals usually have flat faces and sharp edges, whereas biomineral crystals can have strikingly uncommon forms that have evolved to enhance function. The image here was captured using environmental scanning electron microscopy and false colored. Each color highlights a continuous singlecrystal of calcite (CaCO3) made by the sea urchin Arbacia punctulata, at the forming end of one of its teeth. Together, these biomineral crystals fill space, harden the tooth, and toughen it enough to grind rock.