"In nonmetals such as diamond, thermal conductivity is created by lattice vibrations or atomic vibrations of the crystal to transfer energy . The atoms and bonds can be viewed as balls and springs connected together in a chain. When one end is heated, it starts to vibrate causing the next ball and spring to vibrate. This flows down the chain like a wave. The two factors affecting the efficiency of the heat transfer are the coupling between the atoms and how the waves propagate in the crystal and how the waves are scattered due to crystal imperfections. Diamond has extremely strong covalent bonds and so it has a high thermal conductivity [1]. [1] S. O. Kasap, Principles of electronic materials and devices , Third Edition, University of Saskatchewan, Canada."
