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
Rutile is one of five natural forms of titanium dioxide and the most abundant. It sometimes forms like wheat straw or needles within other crystals such as quartz or topaz. Rutile has an adamantine luster and refractive index that rivals diamond but cannot compare in hardness.
Chemical composition: TiO2; Hardness: 6-6.5