Quartz is a crystalline form of silica. The greatest concentrations of quartz are found in silica-rich igneous and metamorphic rocks. Often transparent, Quartz can be found in almost any color. Quartz has piezoelectric properties, meaning it generates an electric charge in response to pressure.
Chemical composition: SiO2; Hardness: 7
Smoky is a variety of Quartz colored by trace elements built into the crystal lattice, in this case, aluminum. Irradiation of aluminum impurities within the quartz produces the grayish to brownish-black color.