A beaver's four incisors grow throughout life. Their outer enamel contains iron-rich mineral inclusions that replace some of the magnesium normally present in the mineral lattice, producing both the deep orange stain and enamel that resists acid better than ordinary enamel.
The cutting geometry depends on a hardness mismatch. Enamel armours only the front surface; the dentine behind it is softer and abrades faster as the animal gnaws. The tooth therefore self-sharpens into a bevelled chisel, and the growth from the root replaces what wears off. A beaver that cannot wear its incisors down develops overgrown teeth that eventually prevent feeding.
The common misreading is that beavers eat wood. They do not digest timber for its own sake. They eat the cambium and inner bark, along with leaves, buds and aquatic plants, and use the felled trunks and branches for dams, lodges and submerged winter food caches.