I think it might have something to with the crystalline structure, but you can have metal crystals and those aren’t clear.
In: Physics
The way an object interacts with light fundamentally comes down to it’s electrons and how they are structured as these are the particles which interact with light.
For a photon of light to be absorbed it needs to interact with an electron and give it its energy. It turns out that electrons in atoms can only take certain values of energy and not values in-between these.
In the simplest case we have hydrogen gas with it’s 1 electron in a certain orbital around the proton, when you pass light through this hydrogen gas it can only absorbs wavelengths of light corresponding to the difference in energy between its orbitals.
For solid materials this is much more complicated but the basic physics is the same, for light to be absorbed it needs to correspond to a transition in electron energy which is allowed by the material. For transparent materials like glass and gemstones there are no transitions which correspond to the wavelengths of visible light that can pass through it.
For crystals which aren’t transparent that means there are transitions allowed which absorb visible light.
Metals on the other hand are somewhat special in that they exist as positive metal ions surrounded by a ‘sea’ of delocalized electrons, in contrast to something like diamond which is made of carbon atoms connected by covalent chemical bonds.
Because of this sea of electrons a metal can interact with basically any photon of light, these photons are absorbed and then almost instantly remitted which results in almost all colours of light being reflected by the metal.
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