Solid-state systems are frequently classi?ed according to their physical, str- tural or chemical properties. Such schemes are extremely helpful since pr- erties related to any such classi?cation are typically known and facilitate id- tifying solids with special material classes. The best-known examples of these schemes are conductivity or resistivity measurements by means of which m- als are easily distinguishable from insulators. However, frequently clear-cut decisions between material classes are not possible, since anisotropy, chemical composition, binding forces and local effects wash out...