Opal is an internationally very popular gemstone that is used in all kinds of jewellery. It belongs to the mineral class of oxides and hydroxides and has a particularly greasy glossy surface. The colour of opal cannot be fixed to a certain spectrum; these minerals can be colourless as well as milky, grey, red, yellow or brown in different intensities. The name "opalus" comes from Sanskrit and means precious stone. There are various sub-types of opals - in general, a distinction is made between the common opal and the precious opal, which is characterised by its multicoloured flashing play of colours. Like a kaleidoscope, it takes on a different appearance depending on the angle from which it is seen. Depending on the colouring, the precious opal is again divided into various groups; the best known of these are white or light opal and black opal. Besides the opalescent precious opals, there is the yellow-red fire opal and the common opal. As the name suggests, the fire opal is usually of a strong red or orange colour. As a rule, the fire opal does not show opalescence, but is rather milky cloudy and only rarely transparent. The common opal is usually cloudy as well and does not show any play of colours. It is also called half opal. Common opals exist in different colours from whitish to yellow and green to brown. Already in ancient times, opals were highly valued because of their characteristic fire. They were even preferred over diamonds. Opals in colourless, weak colouring are most common. Rare are the so-called Pineapple Opals, which are formed in pineapple-shaped pseudomorphs and are therefore particularly sought after. Mexico and Australia are known to be the most common places where Opals are found, but the popular gemstone is also frequently found on all other continents. In jewellery, opals are usually cut into cabochons, because this variation best accentuates the gemstone's play of colours.