Why do iron oxides give rocks and soils red, brown, and black colors?

Iron oxides give rocks and soils their characteristic colors because they exist in various forms with distinct chemical compositions and oxidation states, which determine how they absorb and reflect light.12

The specific color of a mineral or soil depends on the type of iron oxide present:

• Red: Hematite (${\text{Fe}}_2{\text{O}}_3$), the most stable iron oxide on Earth's surface, typically appears red to reddish-black.1
• Brown/Yellow-Brown: Goethite, an iron oxyhydroxide, is responsible for yellow-brown hues and is a key component of many weathered soils.1
• Black/Dark Gray: Magnetite (${\text{Fe}}_3{\text{O}}_4$) typically presents as black or dark gray.1

These compounds function as natural pigments by absorbing specific wavelengths of visible light and scattering others. Because iron can lose or gain different numbers of electrons, these variations in chemical structure allow for the wide range of earthy tones observed in geological formations and soil profiles.31