What is Opal made of? How is it formed? And where does the colour come from?
The known chemical formula of Australian opals is SiO2nH2O,this shows that opals are made of a non-crystalline form of silica that contains some parts water, among other very small trace elements of minerals depending on the ground it’s mined from to produce its overall chemical makeup. Silica is a major constituent of sand which is believed to be the reason as to why Australian opal can produce such stunning colours. This can be explained as the same as when light refracts through a glass prism, the light refracts through the opal and rebounds all levels of the colour spectrum. The most important time in which the formations of opal began in Australia is during Cretaceous period up to the Quaternary period approximately 150 million to 60 million years ago. During these periods there was mass changes happening within the earth’s crust, the sedimentary process in which opal was formed was due to the fact the earth’s crust continued to build layer upon layer. Oceans continued to rise, lakes, rivers and lagoons formed through out various regions of Australia and at one stage a shallow inland sea covered much of inland Australia where majority of opal fields are found today. These rivers and seas that spanned through out Australia allowed for water to seep through cracks and faults in the ground and pool in catchment areas under the ground allowing for opal to form. It’s estimated that it may take up to 5 million years to form 1cm of opal, showing how valuable opal truly is.
Where are opals found in Australia and throughout the world?
Opals in Australia are found in isolated areas where a shallow inland sea stretched from the gulf of Carpentaria, Queensland (QLD) down through New South Wales (NSW) to South Australia (SA) anywhere up to approximately 150 to 60 million years ago. Some of the most noted mining fields in QLD include Eromanga, Yowah, Opalton, Quilpie, and Winton. Queensland mining fields are best known to produce boulder and matrix opal due to the specific rock formation known as ironstone. New South Wales holds one of the most notable mining field areas in‘Lightning Ridge’ which predominately produces the world renown black opal, another area of note in NSW is the White Cliff mining field. South Australia’s highly well-known mining field is Coober Pedy, which produces white and crystal opal, other mining fields include Mintabie, and Andamooka which produces matrix opal. Overall Australia produces approximately 95% of the world’s opals.
Other countries that produce opal include Ethiopia, Brazil, Mexico and Mali for example, however there is varying quality due to the formation and composition of opal found in these regions. For example, Ethiopian opal which forms predominately within weathered layers of rhyolite or volcanic rock which can be porous and more susceptible to water damage.
What type of opals are available within Australia?
In Australia the predominate opals of gem value include the following:
Black opal – Given the name due to having a bar of gem colours over a black or dark background. Black opal is considered one of the most valuable opals available due to the fact the black or dark background helps bring out the brightness and strength in the colours. Black opal is most notably mined in ‘Lightning Ridge, NSW’.
Examples of Australian Black Opal
Black crystal opal – Has a smokey semi-transparent background that delivers high quality colours and value due to having a darker body tone in comparison to crystal opals, this darker body tone helps bring out the vibrant colour spectrum that opals have to offer. Some circles consider black crystal opal alongside the black opal in value and desire due to the ability to bring out the strength and brightness in colours.
Examples of Australian Black Crystal Opal
Grey opal – Varying in colour from light grey to a nearly black background, can also appear smokey or cloudy on occasions. Grey opal sits in between white and black opal in relation to body tone, colours can be very bright when sitting on dense semi-black backings of potch (colourless opal) and provide strong colours that are very desirable.
Examples of Australian grey opal
Crystal opal – Has a translucence, transparent or semi-transparent body which allows one to be able see through or light to shine through the stone. Crystal opal can have very strong memorising colours and can have a body tone varying from black (black crystal) to white (white crystal) opal.
Examples of Australian Crystal Opal
Milky or white opal – An opal with the same gem colour range seen through all other Australian opals but has a milky or white background. White opals don’t have the advantage of the black or boulder opal body tone which enhances the strength of the colour, however there are still fantastic white opal pieces available that has vibrant colour. White opal is most notably mined in South Australia.
Examples of Australian White Opal
Boulder opal – Formed only in Queensland due to the ironstone. Boulder opal is often cut with the ironstone as a backing to allow the stone to be darker and hold the beautiful colour, this makes it a highly valuable type of opal. Boulder opal can display any range of vibrant colours on the colour spectrum and is generally produced in thin veins of ironstone ranging from millimetres to centimetres in thickness.
Examples of Australian Boulder Opal
Matrix boulder opal – Known for its vivid pin flecks of colours, almost looking like stars in the night sky. Matrix is a porous sedimentary ironstone that has minute cavities filled with precious opal that is most commonly found in QLD, not to be confused with Andamooka matrix which is found in South Australia and soaked in a sugar solution then boiled in acid or heated in an oven to give it a darken appearance.
Doublets & Triplets – Is a combination of a piece of opal glued with an epoxy resin to a black backing usually potch (colourless opal) or colourless boulder (ironstone) to allow the piece to be stronger, thicker in size and to bring out the quality in colours that opals supply. When creating a doublet both the rough gem quality opal and rough potch/boulder are adhered together and cut in the same manner as cutting a solid opal. Triplets go through a similar process but also require a glass capping or a clear piece of quartz over the opal layer to help protect the layer of gem quality opal, enhance the colours of the stone and give the piece a cabochon (domed) look. Doublets and triplets aren’t as valuable as solid stones due to having a thinner slice of opal, but it allows for everyone to own there very own piece of this rare gemstone at a fraction of the price, while still supplying beautiful, vibrant colours that opal has to offer.
Difference between solid opal, doublet opal and triplet opal
Unfortunately, opals can be faked, and they’re beginning to appear much more like natural opal. The most common synthetic or man-made opal is known as Gilson opal. Gilson opals are chemically produced in a laboratory using the same components of natural Australian opal, silica,this makes it difficult to detect using a gem tester. Nevertheless, to the trained eye, it can be identified if you look for certain key points. Gilson or man-made opals are generally too perfect or too uniform regarding colour strength and pattern, whereas natural Australian opals has a varying pattern and colour strength throughout the piece. Another give away is that Gilson opal generally has a snake like pattern throughout and will be cut into a perfect oval, circle with a nice cabochon(dome) due to coming out in a man-made block in the lab,in comparison to natural opal which is generally free form due to being produced by nature. However, it’s good to note the shape of the opal is not a key factor in detecting if an opal is a real or not as natural opal can be cut into oval, circle or with a high cabochon if the rock formation allows.
Example of Gilson (fake) opal – Notice the snake like pattern, uniform colour strength throughout and overall too uniform, too perfect.