Why is iron powder black and iron nuggets silvery white?

The free electrons in the metal form a “sea” – the plasma, which determines the metal’s high reflectivity for photons below its plasma frequency (photons above the plasma frequency are absorbed). The plasma frequency of most metals is in the ultraviolet region, so they show luster. Of course, this process is also related to the specific solid energy band. At the same time, the smooth and flat interface guarantees its specular reflection characteristics, so that people can observe the metallic luster. The powder is randomly oriented due to the reflective surface, so the specular reflection is suppressed, resulting in the random orientation of the reflected light, so it will be much duller than the metallic luster. As for why iron is gray, you can check its visible light reflectivity. The reflectivity of iron in the visible light band of 400-700 nm is about 60%, so it is gray. Similarly, our common silver can reflect more than 90% of all visible light and is silver-white.

Flat Iron Bar, for Construction, Rs 30 /kilogram Sri Kanya Corporation |  ID: 13922956488