Diamond from Wikipedia

Electrical Conductivity
Other specialized applications also exist or are being developed, including use as semiconductors: some blue diamonds are natural semiconductors, in contrast to most other diamonds, which are excellent electrical insulators.

Thermal properties
Unlike most electrical insulators, diamond is a good conductor of heat because of the strong covalent bonding within the crystal. Most natural blue diamonds contain boron atoms which replace carbon atoms in the crystal matrix, and also have high thermal conductivity. Specially, purified synthetic diamond has the highest thermal conductivity (2000–2500 W/(m·K), five times more than copper) of any known solid at room temperature. Because diamond has such high thermal conductance it is used in semiconductor manufacture to prevent silicon and other semiconducting materials from overheating. The band gap of diamond is 5.4 - 6.4 eV.

http://en.wikipedia.org/wiki/Diamonds