Ordinary insulating solids, such as diamond, have energy bands that are fully occupied by electrons. The conducting band is so far away from the valence band in diamond that electrons do not have sufficient energy to move -- the 'band gap' is large -- therefore no electric current can be carried.
(http://images.sciencedaily.com/2013/11/131124231802.jpg?1385353313)
source: internet(25.11.13)