That's hot dipped galvanized steel. The zinc crystal pattern formed on it is a called spangling. https://www.steelmillsoftheworld.com/activities/datacenter/G_N

That's hot dipped galvanized steel. The zinc crystal pattern formed on it is a called spangling. https://www.steelmillsoftheworld.com/activities/datacenter/G_Note13.pdf https://www.steelmillsoftheworld.com/activities/datacenter/G... It shines like that, with only some of the crystals being lit up at a given view and sun angle, from the surface being anisotropic. Light reflects more mirrorlike if it bounces parallel to the grooves in each of the individual crystals, and it is scattered if it bounces perpendicular to the grooves. I think there's also an oxide layer contributing some iridescence, essentially a thin layer can cause constructive and destructive interference depending on light wavelength and thickness. If the light bounces mirrorlike, then a fraction of light bounces off the oxide surface, and a fraction transmits, then bounces off the metal. Depending on wavelength, these two light packets will interfere constructively or destructively. It depends on how many extra wavelengths the transmitted light travels (which depends on layer thickness, reflection angle, and light wavelength).