We know that diamond is the hardest naturally occurring substance and so it is much much stable as compared to the other allotrope of carbon i.e graphite as per their structure apparently .But,why then then the bond order of graphite(1.5) is greater than diamond(1).As we know Bond Order is directly proportional to stability.

Graphite has sp2 bonded carbon in planar sheets, whereas Diamond has sp3 bonded carbons.
sp2 has a higher bond energy. Diamond has a greater melting point because of the structure of diamond where each carbon is bonded to four other carbon atoms. In graphite each carbon is bonded to only three carbon atoms in a flat plane. The planes are weakly bound by what amount to London dispersion forces.