Why is diamond a bad conductor of electricity but a good conductor of heat?

Explanation

Conduction of electricity of essentially requires movement of electrons while conduction of heat does not necessarily require movement of electrons. Since in diamond, all the electrons are firmly held C-C, `sigma`-bonds, there are no free electrons in a diamond crystal. Therefore, diamond is a bad conductor of electricity.
In contrast, conduction of heat is due to thermal vibrations from one atom to the other which is easier in a compact crystal of diamond.