The blue topaz is a certain variety of the topaz mineral and is classified as a silicate mineral. Untreated and in its natural state, it offers a high value and has been particularly popular on the international market as a coloured gemstone since the 1970s. The blue topaz forms prismatic, short- and long-columned crystals, is almost glassy with translucent transparency and has a Mohs hardness of 8. Naturally coloured blue topaz occurs very rarely and is of a light blue colour, whereby the colour intensity can vary depending on the region. The colours are classified into three different shades, which include Sky Blue Topaz (sky blue), Swiss Blue Topaz (light blue) and London Blue Topaz (dark blue). Often, colourless or yellow topazes are also dyed blue. This is realised by gamma and electron irradiation, which creates crystal lattice defects that cause a change in colour. They must then be labelled as subsequently coloured blue topaz. Jewellery with blue topaz is very popular, as it is also considered an anniversary stone for the 4th wedding anniversary. The gemstone comes primarily in a facet cut, such as the brilliant or marquise cut, as this is the best way for the topaz to reflect the light and to show off the different shades of blue to their best advantage. The blue topaz is found, for example, in Norway, Madagascar and the USA. Because of its colour, it is often mistaken for blue diamonds and aquamarines.