In Diamond each Carbon atom forms four strong covalent bonds to four other Carbon atoms. This is in the form of a tetrahedron (4 faces).

This whole structure is a strong, rigid collection of atoms, which is why Diamond is both hard and strong. Diamond does not conduct electricity as it has no mobile charged particles. All its electrons are fixed in covalent bonds.

Uses: For making cutting instruments, Jewelry.

Each Carbon atom in Graphite is involved in only three strong covalent bonds which fix the atoms in flat sheets. Each Carbon atom has a spare electron. This is involved in long weak bonds between the sheets. These electrons are delocalised and mobile.

Graphite can conduct electricity by the movement of electrons like a metal, but only in a direction parallel to the sheets.

The reason why Graphite is soft and slippery is because the layers slide across each other due to the weakness of the bonds between them. The bonds in the sheets of Graphite are short and very strong.

Uses: Solid Lubricant, Electrical Conductor.

Quartz Similar to Diamond but has Silicon atoms (Si-O-Si) in the place of Carbon atoms (C-C)