Chiton are marine mollusks who use heavily mineralized and ultrahard teeth to feed on epilithic and endolithic algae on intertidal rocks. To fulfill this function, chiton teeth must be tough and wear-resistant. Impressive mechanical properties are achieved in the chiton tooth through a hierarchically arranged composite structure consisting of a hard shell of organic-encased and highly oriented nanostructured magnetite rods that surround a soft core of organic-rich iron phosphate. Microscopic and spectroscopic analyses combined with finite element simulations are used to probe the ultrastructural features and uncover structure-mechanical property relationships in the fully mineralized teeth of the gumboot chiton Cryptochiton stelleri. By understanding the effects of the nanostructured architecture within the chiton tooth, abrasion-resistant materials can be developed for tooling and machining applications, as well as coatings for equipment and medical implants.