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According to Floris' answer in this link, diamonds are transparent as they have large band gaps while graphite is black as it is a conductor.

As electrical insulators generally have a large band gap, why aren't all insulators transparent?

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    $\begingroup$ If pure without inclusions, defects, scattering centers, and whatnot, they generally will be transparent. For example, sapphire (crystalline aluminum oxide) is wonderfully transparent, while alumina-based ceramics often are not (due to grain sizes and impurities). $\endgroup$ – Jon Custer Aug 17 '17 at 15:23
  • $\begingroup$ @JonCuster That sounds like a fairly complete answer. $\endgroup$ – WetSavannaAnimal Aug 18 '17 at 1:56

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