A remarkable characteristic of nature's hard materials is their ability to resist cracking under stresses and strains. Generally speaking, the secret is their highly ordered, submicroscopic architecture that features a partnership between hard mineral crystals and flexible protein molecules. The chiton tooth's wear and crack resistance is interesting because it's derived from an interactive foursome of carbohydrate, protein, metal ions, and mineral crystal. Magnetite mineral crystals impart the tooth with wear resistance while the carbohydrate, protein, and metal ions organize together to form long, thin fibers imbedded in the mineral crystal; the fibers impart the tooth with crack resistance. The interior of these fibers is an ordered carbohydrate (chitin) scaffold to which flexible proteins are tethered by amorphous chitin strings. The protein molecules, decorating the exterior of the chitin framework, are themselves adorned with metal ions (including sodium and magnesium), which are thought to foster a healthy connection between the fibers and the surrounding magnetite crystals.
"The hardness is notably about 3 times higher than that of enamel and nacre, which exhibit indentation...making this material exceptionally well suited for the continuous scraping activity of the radular teeth. Mechanical mapping of cross-sections through these two regions of the teeth reveals a distinct gradient in mechanical properties with the modulus of the leading edge of the tooth ca. 15% higher than that on the trailing edge. This design strategy results in an uneven wear pattern along the scrapping edge of the tooth and establishes a self-sharpening condition." (Weaver et al. 2010:45)
"This illustrates the important point that structural integrity can be attained only in the presence of the organic matrix that facilitates the anisotropic organization of the magnetite crystallites, binds them into a composite structure, and plays a critical role in crack blunting and deflection at interfaces...Small-scale sliding at the mineral/organic interfaces during external loading is a plausible cause to explain the larger compliance of radular tooth magnetite [compared to a geological magnetite mineral standard]." (Weaver et al. 2010:47)