Group of unworked topaz crystals. Topaz is an aluminium silicate fluoride hydroxide, chemical formula Al2 SiO4 (F,OH)2. It belongs to the orthorhombic crystal system, with the crystal habitat being slimly prismatic, often with a striated surface. Topaz is a popular semi-precious gemstone boasting many colours, ranging from orange- yellow to all hues of blue, pink, colourless and greenish-blue, all with a vitreous lustre. The rough stones here, which are between 20 and 50 mm in length, are from different unspecified locations. Topaz is found in igneous rocks, such as granite and pegmatite, in many places around the world.