What is opal?
Opal is non-crystalline, amorphous hydrated silica (generally forming as spheres) with the chemical formula SiO2 n.H2O.
The “n” means that in between the SiO2 molecules exists between 5 – 20% molecular H2O. The lower the molecular H2O content, the more stable the opal.
Australia produces opal formed in sedimentary deposits with a very low H2O content around 5.5 – 6.5% hence it is stable upon mining. On other continents and locations around the world, the H2O content can be upwards of 10% – 20% and is therefore prone to cracking, crazing and instability.
As the opal forms, it precipitates into spheres which diffract light if arranged in an ordered three dimensional array (precious opal). Australian precious opal therefore is prized around the world as the absolute best due to its vibrant colours & patterns and inherent stability immediately upon being mined – with no treatment or stabilisation processes required.
How is opal formed and mined?
In the sedimentary Australian opal fields, opal forms after water containing dissolved silica seeps into openings and cavities predominantly in clay, then slowly dries out and forms microscopic spheres of silica dioxide. The chemical conditions (for example pH) must be just right for this to occur.
Geologists have many theories about where the silica came from, and how it found its way into the various cavities. The predominant theory for Australia’s sedimentary opal formation is that it occurred predominantly during the Cretaceous period, tens to hundreds of millions of years ago when the earth’s atmosphere was quite acidic. This acidic rain caused feldspar to break down into kaolinite Al2Si2O5(OH)4 where silicon oxidises to become silica – and was called the “deep chemical weathering” theory. Under those conditions, silica becomes soluble in water and can therefore travel downwards via surface cracks in the surrounding sandstone and finds voids to settle in.
The source of sandstone was due to massive flood events along the edges of the ancient Eromanga Sea that deposited unimaginable quantities of sand on top of ancient riverbeds and billabongs. The sand slowly settled and compressed to become sandstone on top of the mud, which also compressed and dried to become clay. As both dried out over millions of years, cracks appeared in the sandstone therefore allowed water to penetrate downwards to settle in any void it found. The voids were created during the initial flood events and subsequent drying periods.
Opal mining therefore is undertaken in areas where the conditions were suitable for highly concentrated silica rich fluids to find their way into any voids, where over time with stable conditions, the clay slowly absorbed moisture from the fluid eventually reaching a precipitation event, and changing state from liquid to solid.
Mining in the early days was with a ‘windlass’ operated by hand, requiring two persons to excavate clay containing opal to the surface. The ‘automatic tipping hoist’ was later developed in the 1960’s to allow for single person mining and is still the predominant extraction method today due to its low cost of operation. Miners who desire to scale up in order to extract significant quantities of ‘opal dirt’ use a ‘blower’ which operates as a giant vacuum cleaner and sucks the opal out of the horizontal ‘drive’ upwards and deposits in a truck, then to be taken to a wash plant for processing.
In the Lightning Ridge and Grawin Opal Fields, two variations of precious opal formed simultaneously:
‘Nobby opal’ is predominant to the east of the dry Coocoran Lake bed including the Lightning Ridge town fields. Nobbies are the result of silica rich fluid entering voids created by trapped air from initial flood turbulence (including organic materials that would have disintegrated leaving spaces behind). The term nobby simply means nodular shape and can be smaller than a pea, with no limits to maximum size.
‘Seam opal’ is predominant to the west of the Coocoran and is the result of fluid depositing into cracks in clay as the mud dried to form clay and contracted naturally, thus opening up veins which similar to nobbies, can be as thin as gladwrap with no limits as to thickness.
In Queensland the silica liquid found its way into ironstone rock (formerly mud ‘pancakes’), and in Coober Pedy in bands within and underneath sandstone.
Where does the colour come from?
Opal is made up of spheres of silica dioxide molecules which are generally half the size of a wavelength of visible light – approx. 150 to 300 nanometres in diameter.
In ‘precious opal’, the spheres are relatively consistent in size and stacked evenly in a three dimensional array. This creates lattices that diffract white light into different colours of the visible spectrum.
These diffracted light waves then travel through the lattice. Depending upon the size of the spheres and how loosely or tightly they are packed, only some wavelengths (colours) are allowed to exit the stone.
Large spheres of 300nm will allow red, orange and yellow to exit but will largely withhold blue and green; and similarly small spheres of 200nm will allow blue and green to exit and largely withhold red. The withholding of light is called ‘destructive interference’.
FUN FACT – Observe any opal in various lighting environments to see how it changes. Opal displaying predominantly red/orange/yellow should look spectacular under a bright white dentist’s light, and a blue/green opal similarly should look amazing in the warm white lighting of a bakery. Different light sources will produce different effects (halogen, mercury vapour, incandescent, fluorescent, LED and sunlight) – the colours you see or not see, are a direct function of the nature of the wavelengths that the light source emits.
Water is important as it provides a medium for light waves to travel through the opal. The 5.5 – 6.5% of water content in Australian opal is sufficient to provide enough medium for light to find multiple pathways out to your eyes – hence the unique ‘play of colour’ as the stone moves or your observation point changes.
What is Potch?
Potch is a colloquial term for ‘opal without play of colour’ (not to be confused by the gemmological term ‘common opal’ usually found in parts of Europe and South America, which display two dimensional pastel hues of generally soft pinks, yellows, greens and blues, but lack the dynamic three dimensional ‘play of colour’ in Australian precious opal).
Potch is comprised of the same silica dioxide molecules, but instead of a regularly stacked three dimensional array, they are of misshapen shapes and sizes – irregularly shaped and jumbled (like gravel poured into a container), so they cannot diffract light. Hence potch is typically black, grey or white and opaque.
Potch blackness is influenced by trace elements and the potential presence of pyrite and chalcopyrite.
Potch is a vital component when formed naturally and simultaneously with precious opal. When precious opal forms homogeneously on top of a layer of potch, the opal base becomes opaque therefore concentrating all light emanating from the stone via the dome. This creates a depth, contrast, brilliance and patterns not seen anywhere else on earth outside of Lightning Ridge.
