According to the Australian aboriginies--Australia being the preeminant source of opals--the gems are the physical manifestation of a rainbow that brought peace to all humans.
Precious opals display an optical effect called ‘play of colour’ or ‘fire’. This is due to a combination of different factors. Opal is composed of a stacked framework of silica spheres, and it is the diffraction and interference of light between these spheres that create the fascinating colour play. The colours produced depend on the size of these spheres. A whole spectrum of colours can be seen in the most desirable opals. The play of colour shimmers with iridescence across the stone and is distinctive of these beautiful gems.
Opals occur in a range of body colours from white, black or grey, bright orangey red and a pale watery colour. The most precious opals show strong colour contrast and generally have a dark body colour with a vivid array of colour play. They are usually cut en cabochon which displays their optical effect beautifully and is also the best shape for their hardness.
In Roman times opals were mined in Czechoslovakia, and continued for centuries to produce white opals, prior to the discovery of the world’s main source for opals, Australia. Australia is famous for its opal production of the most vivid colours and occurs in many localities. In 1903 black opal was found in Lightning Ridge in north-east New South Wales, which is now one of the most famous areas for precious opals in the world. An opal weighing 203 carats was found in Andamooka, Australia, in 1949 and was set into a necklace presented to Queen Elizabeth II. Mexico is another major source for opals, especially for fire opal of a strong red / orange body colour.