Uniformitarianism is a geological doctrine. It states that current geologic processes, occurring at the same rates observed today, in the same manner, account for all of Earth's geological features. Based on this theory, the majority of geologists believe that opal is formed in a slow and gradual processes over millions of years, and that opal formation is a sedimentary process. It is believed that silica gel (a warm water solution supersaturated with silica) was deposited over millions of years, layer after layer, filling in cracks and spaces in parent rock, often a friable sandstone. This type of sandstone is commonly called opal dirt by miners. It is thought, that over millions of years the gel slowly dries out to become hard. It has been suggested that it takes around five million years for approximately 1 centimetre (a little more than a third of an inch) of opal to develop, perhaps through rain washing the silica gel into friable sandstone.
The other theory on opal formation is the creationist view which is that given the correct environment opal can be formed within a much shorter space of time. Opal has been formed in jars by Dr Len Cram in a period of months rather than millions of years. The CSIRO have credited Dr Cram with the most realistic laboratory opal in the world, surpassing any synthetic opal that they attempted to make. The CSIRO, whilst having conflicting views, recognise Dr Cram as one of the world's most prominent scientists on the subject of opal formation. The opal that is naturally formed by Dr Cram in his laboratory is so realistic that many experienced miners cannot tell the difference between opal dug from the ground and opal formed in the lab. We have more information about Dr Len Cram on our blog titled "How opal is formed - creationist theory"
Opal formed in jars by Dr Len Cram
Formation of precious opal
Precious opal is comprised of small spheres of amorphous silica arranged in a regular pattern. White light is diffracted by these layers and broken up into the colours of the spectrum, causing the characteristic display of colours for which opal is so highly prized. The body colour or background for the diffracted colour play may be milky white, grey, blue, black or colourless. Black opal is the most highly sought after variety as the dark background shows off the colour play best.
Formation of common opal
In common opal or potch, which shows no play of colours, the silica spheres are either made up of assorted sizes which cannot form a regular structure or are too small to allow the light diffraction required to produce a visible play of colour.
Precious opal vs Potch opal
Each local opal field or occurrence must have contained voids or porosity of some sort to provide a site for opal deposition. In volcanic rocks and adjacent environments, the opal appears to fill only voids and cracks, whereas in sedimentary rocks there are a variety of voids created by the weathering process. Leaching of carbonate from boulders, nodules, many different fossils, along with the existing cracks, open centers of ironstone nodules and horizontal seams provide a multitude of molds ready for the deposition of secondary minerals such as opal.
Opal deposit filling a void in the sedimentary rock layer
The many variations in the types of opal depends on a number of factors. In particular, the climate provides alternating wet and dry periods, creating a rising or more importantly a falling water table which concentrates any silica in solution. The silica itself is formed either by volcanic origin or by deep weathering of Cretaceous clay sediments producing both silica and white kaolin often seen associated with the Australian opal fields. Special conditions must also prevail to slow down a falling water table in order to provide the unique situation for the production of its own variety of opal.
The chemical conditions responsible for producing opal are still being researched, however some maintain that there must be acidic conditions at some stage during the process to form silica spheres, possibly created by microbes.
Volcanic opal generally contains more water than sedimentary opal, and when mined from fresh lava, has a tendency to dry out and crack. With the exception of some Mexican fire opal, most volcanic opal crazes, Ethiopian Welo opal is a classic for this and my experience has been it gets more cracks and crazing in it than an elephants bum.
Opal with light underneath to highlight cracks
While volcanic-hosted and other types of precious opal are found in Australia, virtually all economic production comes from sediment-hosted deposits associated with the Great Artisian Basin.
Australia has three major varieties of natural sediment-hosted precious opal – black opal from Lightning Ridge in New South Wales, white opal from South Australia and boulder opal from Queensland.
Formation of black and white opal
White and black opal is formed as described above. The colour tone of Australian black opal can range from dark grey to jet black and is caused by the presence of carbon and iron oxide trace elements.
White Coober Pedy opal
Lightning Ridge black opal
Formation of boulder opal
Queensland boulder opal forms in a slightly different method to other types of opal, forming inside an ironstone concretion. The concretion was formed due to ionisation, from sedimentary deposition. By definition, they are ionised concretions of varying hardness with an approximate opal composition of SiO2at 28%, Fe2O3 + AL203 at 68% and H2O at 1% composition.
The opal forms in generally elongated or ellipsoidal ironstone concretions (boulders) ranging from a few centimeters up to 3 m across. The boulders may be confined to one or more layers or randomly distributed through the weathered sandstone. Their composition ranges from sandstone types (a rim or crust of ferruginised sandstone surrounding a sandstone core) or ironstone types (composed almost entirely of iron oxides).
QLD boulder opal
The opal occurs as a filling or lining between the concentric layers or in radial or random cracks in the ironstone, or as a kernel in smaller concretions known as nuts (as found at Yowah and Koroit fields, the famous 'Yowah-nuts').
Matrix boulder opal is where the opal occurs as a network of veins or infilling of voids or between grains of the host rock (ferruginous sandstone or ironstone). Rare seam or band opal is also found and is typically encased in ironstone.
Boulder opal with opal veins
Boulder matrix opal (infilling of opal between grains of host rock)
Pipe opal occurs in pipe-like structures which may be up to several centimeters in diameter within the sandstone and these structures may be hollow or opal-filled. Wood opal is occasionally found replacing woody tissue material.
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