Diamond has three dimensional network of strong covalent bonds. It is very difficult to break extended covalent bonding and therefore, diamond is the hardest substance on earth. On the other hand, graphite has layer structure and the layers are held together by weak Vander Waals\x92 forces. Therefore, graphite cleaves easily between the layers and, therefore, it is very soft and slippery.
It is because of the structural differences in them
Yes , it is true .Diamond is hardest non metal and graphite is one of the soft non metals