Sea urchin teeth have many plates of crystalline calcite. The intricate strategy by which sea urchin teeth develop evolved to fill space and resist fracture. This false-colour scanning electron microscope (SEM) micrograph shows the forming end of a tooth from the Florida sea urchin Arbacia punctulata. Each tooth element is highlighted by a different colour, and is a single crystal of calcite (CaCO3). Unlike geologic crystals, these biomineral crystals do not have flat surfaces and sharp edges, but show curved surfaces.
Image courtesy of Pupa Gilbert, University of Wisconsin-Madison (2011).