Iron oxide may be part of one of humanity's oldest chemical technologies: making color.
Prehistoric people processed iron-rich ochre tens of thousands of years ago. Iron minerals can appear yellow, red, brown, or black depending on oxidation state, structure, hydration, and particle size.
Heat can transform yellow iron oxyhydroxides toward red hematite.
From prehistoric ochre to modern pigments, the principle remains: structure determines color.
Global Chemical Database