A chiton feeds by dragging its radula across rock to scrape off algae, and rock wins against soft tissue. The animal answers by mineralising the leading teeth. Each tooth begins as an organic scaffold, which is first filled with ferrihydrite and then converted to magnetite, giving a cap that is among the hardest known biominerals.
The teeth are produced in a conveyor: new teeth form at the rear of the radular ribbon and mature as they advance forward, so a fully hardened tooth is always in the working position while worn ones are shed. Cryptochiton stelleri has become a standard subject for studying this process because it is large enough to sample each maturation stage along the ribbon.
The cautions are worth naming. Magnetite is magnetic, but there is no good evidence that chitons use their teeth for magnetoreception; the mineral is chosen for hardness. And the chiton is not unique in mineralising teeth, only unusual in the mineral it selects, since most animals reinforce teeth with calcium phosphate or calcium carbonate instead.