Noun
Pure silica, like quartz, but crystallizing in hexagonal tables. It is found in trachyte and similar rocks.
Source: Webster's dictionaryThe structures of tridymite and cristobalite are closely related and can be described as hexagonal and cubic close-packed layers. Source: Internet
The high temperature minerals, cristobalite and tridymite, have both a lower density and index of refraction than quartz. Source: Internet