Sea urchins bore through rock to carve out nooks, where they can protect themselves from waves and predators. To learn how urchin teeth stay sharp despite all that grinding, researchers analyzed the roughly 0.8-inch-long (2-centimeter-long) choppers of the California purple sea urchin.

(Related: "Sea Urchin Genome Reveals Striking Similarities to Humans.")

Using high-resolution x-ray imaging, the investigators found that the teeth are mosaics of two kinds of calcite crystals: fibers and curved plates. The crystal shapes are arranged crosswise to each other and are bound together with a superhard cement of calcite nanoparticles.

Between the crystals are layers of weaker organic material. By striking the teeth with microscopic, diamond-tipped probes, the scientists found