Topaz is the name given to a family of chemically related stones Al2(SiO4)(F,OH)2. With a hardness of 8 on the Mohs Scale they are considered precious gemstones. Topaz comes in numerous colours including colourless, white, pale blue, light green, yellow, yellowish brown, or red. The bright blues so popular for jewellery are nearly always heat treated to make them more intense in colour. Natural deep blues are rare to come across, as are many of the other colours, and these can be highly valued. All types of Topaz have a vitreous lustre, a specific gravity of 3.4 – 3.6 and form according to a orthorhombic crystal system.