The best material that serves as an insulator is glass because of its high resistivity, states Georgia State University or GSU. The photons present in glass do not have enough energy to close the gap between the valence electrons and the conduction band.
Other materials that serve as insulators include mica and quartz. Solid materials act as good insulators by restricting electric current flow, but are bad conductors, explains Georgia State University.
Two other types of insulating materials are conductors and semiconductors. Conductors allow electrons to move freely throughout the material. Most metals are both good electrical and heat conductors, notes Georgia State University. Additionally, the energy bands within semiconductors allow some electron movement and electric flow.