Graphite is good conductor of electricity but diamond is non-conductor because

→ In graphite each carbon atom undergoing sp² hydridisation and form hexagonal layer structure. The unhybridised P-electrons of adjacent carbon atom form mobile π-electron cloud and hence graphite acts as conductor
→ Where as in diamond the carbon undergoing sp³ hybridisation, and froms four bonds tetrahedrally with other 4-carbon atom. Hence here no unpaired electron in diamond to show conductivity