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 found in igneous rocks, like granite and pegmatite, in many places around the world. When clear and without impurities topaz is a popular semi-precious gemstone that comes in a large variety of colours. The specimen seen here is a rough crystal group from a further unspecified locality in Siberia. Actual width is 75 mm