High-temperature polymorphs of this mineral include tridymite and cristobalite.
High-temperature polymorphs of this mineral include tridymite and cristobalite. Despite their name, Herkimer diamonds are actually crystals of this mineral. The composition of igneous rocks is shown on a double triangle diagram named for this mineral, alkali feldspars, plagioclases, and feldspathoids. Located at the bottom of Bowen's reaction series, this mineral is found in between orthoclase and topaz on the Mohs hardness scale. Meteorite impacts cause the "shocked" type of this mineral. The gemstone amethyst is a violet variety of this mineral, and sand is composed of small fragments of it. For 10 points, name this mineral that is a form of silicon dioxide. 
High-temperature polymorphs of this mineral include tridymite and cristobalite.
quartz