Topaz is an aluminum silicate mineral of composition Al₂SiO₄(F,OH)₂, forming prismatic crystals in the orthorhombic system which are variable and unstable in color, and valued as gemstones. The best topazes come from Brazil, Siberia, and the United States.

Topaz belongs to the orthorhombic system. The unit cell is a straight prism on a rectangular base.