Question

Why diamond is hard but graphite is smooth and slippery?

Answer 1

Graphite contains layers of carbon atoms. The layers slide over each other easily because there are only weak forces between them, makinggraphite slippery. Graphite contains delocalised electrons (free electrons).
hope I helped you

Answer 2

it is true that they both are made up of carbon atoms but there is different arrangement of carbon atoms in diamond and graphite that's why their chemical properties are same but physical properties are different.