Pure metallic iron is silvery white, and iron powder is black. This is an optical problem because the surface area of iron powder is small and there is no fixed geometric shape. The crystal structure of iron block is geometrical, so the iron block absorbs part of visible light. The other part of the visible light is mirror-reflected and appears white; the light that the iron powder has not absorbed is diffusely reflected, and the visible light that can enter the human eye is small, so it is black.