Topaz, Al₂(SiO₄)F₂

Topaz, Al₂(SiO₄)F₂, is a fluosilicate of aluminium. It is an orthorhombic mineral (a:b:c = 0.528:1:0.477), and its hardness (8), high refractive index, and variety of colour render it valuable as a gem-stone. The usual colour is pale yellow, but light brown, dark brown, and pale blue crystals are also found.