Most diamonds are electrical insulators and extremely efficient thermal conductors. They are materials that have properties that are intermediate between conductors and insulators. Most insulators (or dielectric materials) are held together by ionic bonds. These hollow insulators with a conductor inside them are called bushings. Stephen Gray discovered the importance of insulators and conductors. Semiconductor materials are nominally small band gap insulators. It is an electrically neutral quasiparticle that exists in insulators, semiconductors and some liquids. Some magnetrons have ceramic insulators with beryllium oxide (beryllia) added. Suspension structures support the conductor vertically using suspension insulators. It is also used in electronics, electrical insulators and abrasives. The thermal conductivity of BN is among the highest of all electric insulators. Radio-frequency has ability to appear to flow through insulators (non-conductive materials). This idea was to replace ceramic with plastic insulators. The distinction between semiconductors and insulators is a matter of convention. The thermal conductivity of BN is among the highest of all electric insulators (see table).