Silica group - Group of silicate minerals (tectosilicates) composed only of silica (silicon dioxide -- SiO₂ ). The minerals in this group are all the varieties of Quartz, Chalcedony, Opal, Tridymite, and Cristobalite (and a few rarer forms of silicon dioxide). Although Opal contains water in addition to silica, it is nevertheless in the silica group.
The Silica Group is also known as the Quartz Group.

See Polymorphs of SiO2 for more detailed information on the minerals in the silica group.