Agate is a common semiprecious silica mineral, a variety of chalcedony that occurs in bands of varying color and transparency. Agate is essentially quartz, and its physical properties are in general those of that mineral. Agate is a variety of chalcedony, which is a mineral in the quartz group. The chemical composition of agate is primarily silicon dioxide (SiO2), with small amounts of impurities and trace elements that give it its unique color.