It may be prickly on the outside, but the sea urchinâ??s spines hide â?? well, if not a heart of gold, at least teeth of calcite.
The urchinâ??s five teeth are very strong and capable of grinding limestone, creating depressions in the rock that the sea urchin can settle in. Since limestone also consists of calcite (a form of calcium carbonate), how can the teeth grind the rock without being ground down, too?
The teeth consist of calcite crystals (including a small amount of magnesium) in the shapes of needles and plates, and a polycrystalline calcite matrix that contains a higher concentration of magnesium. Yurong Ma, Lia Addadi and Steve Weiner of the Weizmann Institute of Science in Israel, with Pupa Gilbert of the University of Wisconsin and other colleagues, show that the magnesium levels increase toward the tip, contributing to the hardness, and that all the elements are aligned in what they call polycrystalline blocks.