Glass.

Glass; Middle and Old High German glass, originally = “amber”; actually = “shimmering”, “shiny”, related to “yellow”

A solid, in its mass non-crystalline, brittle, inorganic material [...], that has no definite melting point, but when increasingly warmed will pass without sudden change in its properties into a softer and finally liquid state.
Source: Brockhaus.de

Without glass there would be no windows. The Finstral insulation glass works in Oppeano (near Verona), Scurelle (Trentino) and Gochsheim (Lower Franconia) are among the most modern in Europe. Here, the standard glass panes, up to 19 square metres in size, are cut to size in a fully automatic, highly efficient process, the edges are finished and, if desired, the glass is hardened to make it extremely resistant. Following an uncompromisingly thorough quality inspection using a laser scanner, up to three panes are assembled together to form the finished insulation glass. The spacers too are also of very high quality: they are thermally insulated, mitred and corner welded. This is for example how the most modern triple glazing currently available in Europe is created: Max-Valor combines the insulation of triple glazing with the high light transmittance of double glazing.