Bare rock does not soften into metal; the sky edits it. At low solar angles, sunlight plows through an extended column of air, where Rayleigh scattering strips out much of the short wavelength blue and violet portion of the spectrum, so the beam that finally reaches the cliff is already skewed toward red and orange energy.
The harsh truth is that the rock’s spectrum barely budges while the atmosphere behaves like a vast optical filter. Aerosols add Mie scattering to the mix, nudging more green light aside, while dust and water vapor absorb selectively across wavelengths, so the reflected signal that bounces off quartz, feldspar, and iron oxides and into human retinas carries an inflated share of long wavelength photons compared with midday illumination.
What changes first is perception, not geology. Cone photoreceptors adapt to the dimmer, red weighted field, and color constancy algorithms in the visual cortex quietly fail under such skewed spectral power distribution, so a surface that measured as beige or gray under full spectrum light is recoded by the brain as copper, brass, even molten gold, though no atom in the desert has moved.
