Quartz. Molecular model of quartz, one of the most abundant minerals in the Earth's crust. Quartz is a crystalline form of silica (silicon dioxide, SiO2). A crystalline structure consists of a repeating unit cell. Quartz has a unit cell of three silicon atoms (black balls) and six oxygen atoms (red balls). In the overall structure, each silicon atom forms bonds (sticks) with four oxygen atoms and each oxygen atom forms bonds with two silicon atoms. Crystals of quartz can now be produced synthetically and are mainly used in optical and scientific instruments and in electronics, such as quartz watches.