Diamond is a semiconductor. In order for electrons to conduct electricity, they must overcome an energy barrier known as the band gap. When the electrons obtain energy from an applied voltage, they can jump to the conduction band. This is how semiconductors and insulators conduct electricity,their electrical conductivity is low (< 100 mho/ cm). Generally semiconductors have band gaps around 5 eV (1 eV = 1.6x10e-19 J) or less. Hence, since diamond has a band gap of 5 eV, it is considered a wide-band-gap semiconductor.toms are tetrahedrally coordinated by 3 other carbon atoms.Electricity is conducted in a crystal by electrons that are relatively free. But in diamond, each carbon atom is covalently bonded with four other carbon atoms and there are no free electrons.
It is bad conductor of electricity because no free electrons are left to pass electricity