Each of its oxygen atoms is part of two tetrahedrons.
High temperature polymorphs of this mineral include tridymite (TRIH-dih-mite) and cristobalite (KRIS-toe-ball-ite). The alpha form of this mineral has a trigonal structure, while its beta form is hexagonal. When microscopic crystals of it are intermixed with mogánite, chalcedony (chall-SEE-dun-ee) results. Citrine is a yellow colored type of this mineral, and (*) amethyst is a purple one. Each of its oxygen atoms is part of two tetrahedrons. This mineral exhibits the piezoelectric effect. For 10 points, name this mineral that has an overall formula corresponding with silicon dioxide, and has a hardness of seven on the Mohs scale.
High temperature polymorphs of this mineral include tridymite (TRIH-dih-mite) and cristobalite (KRIS-toe-ball-ite).
{quartz} [prompt on {silica}; prompt on {silicon dioxide}; prompt on {SiO2}]