You know already that light interacts with the charged particles in the atoms. The main question to ask to determine the optical character of a particular material is: What states are available for electrons to occupy? This question can be answered by the band structure or energy levels of the material/molecule, which depend on the nature and geometry of the bonding between atoms as determined by quantum mechanics.
If there are no states available for the electrons, there can be no absorption. Diamond is an insulator because its crystalline geometry determines that it has a large band gap, and visible light simply does not have enough energy to promote electrons to the available upper states. So the electrons stay where they are, and light is primarily transmitted.
In contrast, graphite has a different crystalline geometry. The result is that it has no band gap whatsoever. So no matter what color of light you choose, there will be states available for the electrons to be excited. So the electrons will absorb the light power, going into the excited states, and the light will be attenuated.
A metal has lots of states at low energies that electrons can move into. Once again, this is determined by band structure. Since there are lots of low energy states, the electrons can basically just slosh around with the driving fields. This makes metals good conductors.
