Diamond, analogy to methane

In methane the bonding network of the carbon atoms is terminated by the H atoms.  Because the H atoms can only form one covalent bond, which is confined within the molecule, there is no possibility for extended covalent bonding between different molecules.  The inter-molecular bonds must therefore rely on weaker dispersion forces for their cohesive forces.

In diamond, which can be constructed by replacing the H atoms in methane by other C atoms, the bonding is not terminated and the s sp³ bonds between the C atoms are extended throughout the solid.  These are strong bonds, as a result diamond has a very high melting (>3550°C) and boiling (4827°C) point. (for a more detailed description of the bonding go to  Conduction properties)
 

Methane	Crystal Structure of Diamond

 

 
 The crystal strucutre of diamond is comprised of tetrahedrally coordinated C atoms.  The unit cell is fcc, with 2 carbon atoms per lattice point, and 8 atoms per cell.
The diamond structure is also adopted by several well known semi-conductors such as Si, Ge (and also gray Sn).