"[[Semiconductor|[S]emiconductor]]s as are distinct from s, yet the properties of both are readily explained by the same band theory. ...[T]he semiconducting properties of this metalic-appearing silicon and the absence of color and insulating properties of, say, diamond, are both part of a continuous range of materials which unexpectedly includes the red, orange, and yellow colors of medium band-gap semiconductors. ...It is the delocalized nature of the s, their ability to move freely throughout a piece of metal or semiconductor, which is at the root of the characteristics of these two types of material."