The chiton has a radula covered with magnetite teeth that scrape algae from rocks. A radula is a ribbon-like feeding structure found in molluscs, and chiton teeth are reinforced with iron-bearing minerals, making them among the hardest biological materials known.
Chitons are flattened marine molluscs with eight overlapping shell plates. They cling tightly to rocks in intertidal and shallow subtidal habitats, using a muscular foot and a girdle around the shell plates. As they graze, the radula repeatedly moves across rock surfaces to remove algae and other food.
Abalones and limpets also use radulae to graze, which makes them frequent answers to this question. The distinctive clue is magnetite: chiton teeth contain a particularly high concentration of iron minerals. Their teeth are continuously replaced as the front rows wear down.