Diamond is hard and graphite is soft, why?

In Diamond. each carbon atom is covalently bonded with four other carbon atoms along the four corners of a regular tetrahedron in three dimensional. Therefore diamond is hard due to strong covalent bonds present in it. Whereas in graphite, carbon atoms are bonded together in flat layers by strong covalent bonds in a regular hexagon. But these layers are held together by Van der Waal's forces which are much weaker as compared to the covalent bonds. This is the reason why graphite is soft and slippery.