Diamond is harder than graphite because it is a crystalline form of carbon in which each carbon atom is covalently connected with four other carbon atoms.
The carbon atoms in a diamond form 4 covalent bonds as a tetrahedral structure.
Graphite is an amorphous allotrope of carbon.
Where the carbon atoms in the graphite form 4 covalent bonds as a hexagonal structure. It forms layers, which can slide over each other.
Here the force of attraction between the layers is weak, and this is the reason why they can slide over each other.
Graphite is also soft and slippery.
They are used in the industries in the form of lubricants.
