First I had the billiard balls all diffuse. This did not really look good in the final rendering, because all balls appeared to matt. Therefore, I implemented also the conductor.
Following properties will be available for the texture to add the texture:
eta
k
I decided to set the eta and k parameters directly in the XML. It is an even better extension if one could simply specify the type of the conductor as a material-string. However, since it was only to improve the final image, I stayed with this first draft, which already makes what I wanted.
This feature I am going to test simply by reasoning about the appearance. For me, it looks as it is expected. By setting the BSDF to a conductor, the appearance becomes shiny. It is also not too bright and does also not darken too much, based on the values. Therefore I think it looks pretty accurate.