Contrary to a popular misbelief, a single covalent bond is much stronger than a single ionic bond. While the energy of an average ionic bond is in the range of 4-7 Kcal/mole, that of a covalent bond is 80 Kcal/mole. Network covalent solids like diamond and silicon dioxide are made up of a throughout three dimensional covalent bonded structure and hence bound by pure covalent bonds in all directions (intramolecular as well as intermolecular). That gives extraordinary stability to these solids. This further explains why world’s strongest substances are all covalent network solids. The same reason explains there extraordinarily high melting and boiling points as a huge amount of energy has to be invested in order to break some of these intermolecular bonds. As all the electrons are either bound by covalent bonds or within the atoms, in lone pairs there is no free or labile electrons within these solids and hence they are all non-conductors of electricity and heat.