There are two effects that come into play: the way light reflects off of the surface (surface reflection) and the way light bounces into and then out of the surface (subsurface reflection). Surface reflection is responsible for the darkening of wet or polished stones. The interesting colors of stones come, for the most part, from subsurface reflection. Different materials absorb light of different colors, and allow others to pass, so light that undergoes subsurface reflection picks up some colors. With a polished stone, that white light is being reflected away in one direction, so unless you’re in the path of that white light you’ll just be seeing the more colorful subsurface-reflected light.