Ceramics: Germanium dioxide
Typically classified as an inorganic compound, germanium dioxide (GeO2) has two polymorphs, crystallizing in the hexagonal and tetragonal crystal systems, and is capable of solidifying in a amorphous structure composed of GeO4 tetrahedra. (The tetragonal crystal structure is pictured above.)
Because of its high reactivity, GeO2 is not considered a naturally occurring material, found instead in other ores and minerals such as zinc ores. It can also be found in the form of a passivation layer on pure germanium materials exposed to air.
This material is used primarily in optical applications, such as certain lenses and optical fibers, as well as applications that take advantage of the fact that it is transparent to infrared light. Other applications include as a catalyst, or in the production of other germanium compounds.
Sources/Further Reading: ( 1 ) ( 2 - images 1 and 2 ) ( 3 )
