Magnetite crystallizes in an inverse spinel structure containing both ferrous (Fe²⁺) and ferric (Fe³⁺) iron; naturally magnetized specimens known as lodestone enabled the earliest magnetic compasses.
As basaltic lavas cool below magnetite's 580 °C Curie temperature at mid-ocean ridges, magnetite grains lock in the orientation of Earth's magnetic field, preserving the seafloor magnetic striping that proved plate tectonics.
Biomineralized magnetite nanocrystals (magnetosomes) are synthesized by magnetotactic bacteria and migratory birds for geomagnetic navigation, while reduced magnetite serves as the industrial catalyst for the Haber–Bosch ammonia process.