The gem is known in Lightning Ridge and White Cliffs, New South Wales; Mintabie, Coober Pedy; Andamooka, South Australia; and in several areas in Queensland such as the Quilpie district, Yowah district and the Winton district. A small quantity is also produced in Brazil. Mexico and the US state of Oregon produce a volcanic opal called fire opal – a gem with a body colour of oranges and reds.
Opal varieties are determined by the specimen's origin and colour. Classifications include dark or black opal, white or light opal, milk or crystal opal, boulder opal, matrix opal, Yowah Nuts from Queensland (many known as “picture stones”) and Mexican and fire opal.
Different opals have differences in mineral composition depending on the geological environment from which they originate.
Opal is set apart from other gemstones because of its array of vibrant colours, which change and flash as it is turned. Precious opal is classified on the basis of its body tone – the relative darkness or lightness of the opal and the type of colour pattern or play of colour. Black opal shows a play of spectral colours in dark body. Light, fire or crystal opal has body colour ranging from milky white to translucent. Boulder opal can be light or dark, found in veins of ironstone – the host rock in which it forms. This variety often provides a good dark background and displays a bright and vibrant play of colour.
Body tone, colour play and pattern are all equally important in determining an opal's value.
A black opal showing a very bright, red play of colour in a special pattern is the most valuable.
The thickness of the opal layer is also a consideration, as is the beauty of the patterning, the cut, weight and finish.
In the 1960s, a team of Australian scientists working for the CSIRO analysed opals with an electron microscope to determine the cause of the gem's unique colour play. They discovered that small spheres from silica gel caused interference and refraction manifestations. The spheres succeed in dissecting the light on its passage through the gemstone and turning it into all the colours of the rainbow.
Opal's name evolved from the word opalus. Used by the Romans in the first century BC, opalus is said to be derived from the Sanskrit word upala, which simply means "precious stone".
The gems are said to rejuvenate emotional and mental forces and assist with competency and efficiency, fostering creativity and originality.
An Aboriginal dreamtime legend has its own explanation for the creation of opals. It believes that the creator came to earth on a rainbow, to bring the message of peace to all of human kind.
When his foot touched the ground, the stones were awakened and started sparkling in every colour of the rainbow.
Shakespeare found opal a symbol of shifting inconstancy, likening play of colour to play of mind in one of the most apt uses of gemstone symbolism in literature. In Twelfth Night, he wrote: "Now the melancholy God protect thee, and the tailor make thy garments of changeable taffeta, for thy mind is opal." Opal is the anniversary gemstone for the 14th year of marriage and the modern birthstone for October.
Hardness: 5.5 to 6.5
Variety of: opal is a form of hydrated silica(SiO2.nH2O) and precious opal usually contains six to 10 per cent water.
Found: Australia, Brazil, USA and West Africa.
Traditional birthstone: Tourmaline
Mystical birthstone: Jasper
Ayurvedic birthstone: Opal
Libra (September 23- October 22) opal, lapis lazuli, peridot
Scorpio (October 23- November 21) aquamarine, topaz