Two varieties of this mineral formed at high pressure and found in some meteorite craters are called coesite and stishovite. One variety's brown-gold fibrous crocidolite inclusions exhibit chatoyancy. That effect, also called "cat's eye", may be present along with a diasterism when this mineral is backlit. At the bottom of 
Two varieties of this mineral formed at high pressure and found in some meteorite craters are called coesite and stishovite. One variety's brown-gold fibrous crocidolite inclusions exhibit chatoyancy. That effect, also called "cat's eye", may be present along with a diasterism when this mineral is backlit. At the bottom of Bowen's reaction series, this trigonal crystal displays piezoelectricity, the basis for its use in clocks. It includes onyx, citrine, and "rose" varieties. For 10 points, name this mineral with chemical formula SiO2 and Mohs hardness of 7, which, when purple, is called amethyst.
Two varieties of this mineral formed at high pressure and found in some meteorite craters are called coesite and stishovite.
quartz [prompt on silicon dioxide; prompt on SiO2]