Question: why is the standard enthalpy change of formation of diamond not zero?
Answer: Although diamond is made up of single element but every carbon atom is connected with 4 different carbons which causes it's enthalpy of formation non zero. Elements in their standard state are not formed, they just are. So, ΔH°f for C (s, graphite) is zero, but the ΔH°f for C (s, diamond) is 2 kJ/mol.