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These images show a collection of simple and common material configurations, inspired by  Weidlich and Wilkie's arbitrarily layered micro-facet surfaces. These have been rendered with both the fresnel blend mode, and rough coating mode.

Smooth gold with a rough/bumpy purple coating

These images show a smooth gold base layer with a bump mapped dielectric with absorption.

In the fresnel blend image, the effect of the bump map is isolated to the top dielectric layer, as the rays that reach the base layer only pass through the top layer and are not refracted. In the rough coating image, we can see that the bump map has a much larger effect on the base layer as the rays are being refracted by the top layer.

The absorption also works differently, giving less variation in colour across the surface in the rough coating case. This is because the refraction causes the distances that the rays travel between the top layer and base layer to be compressed into a smaller range.

We can also see that in the rough coating image, the base layer reflection is much more diffused, this is due to the scattering of rays through the top layer, which is achieved by the roughness approximation.