Question

Graphite and diamond both are allotropes of carbon, still graphite acts as a lubricant and is a good conductor of electricity, but diamond is hard and does not conduct electricity.

Answer 1

Graphite has one electron unpaired and it's structure has layers which make it slippery and good conductor of electricity.

Answer 2

$graphite undergoes sp2 hybridization,so the molecules are held through weak vanderwaals forces and act as lubricant
$where as for diamond it undergoes sp3 hybridization,so the molecules are held through covalent bonds and is hard