In terms of thermal conductivity (k) and electrical conductivity (δ) diamond has

Diamond exhibits a unique combination of extreme physical properties. It possesses a exceptionally high thermal conductivity ($k$) due to its rigid, strongly bonded covalent 3D tetrahedral network, which allows lattice vibrations (phonons) to propagate rapidly. Conversely, all of its valence electrons are locked firmly in localized covalent bonds, leaving no free mobile charge carriers. This results in an incredibly low electrical conductivity ($\delta$), making it an electrical insulator.