The cryptocrystalline variety of this mineral is called chalcedony.
This mineral shares Nickel-Strunz group 4.DA with minerals like coesite and stishovite, which have identical chemical compositions. Herkimer diamonds are actually doubly-terminated crystals of this mineral, which crystallizes at lower temperatures than muscovite and is found at the (*) bottom of Bowen's Reaction Series. The cryptocrystalline variety of this mineral is called chalcedony. This piezoelectric material is the second-most-common in the Earth's crust after feldspar, and has Mohs hardness of seven. For 10 points, name this mineral made of silicon dioxide, one variety of which is amethyst.
This mineral shares Nickel-Strunz group 4.DA with minerals like coesite and stishovite, which have identical chemical compositions.
{quartz} [prompt on {silicon dioxide} or {SiO2} before mentioned]