The black opal mined at Lightning Ridge is a unique and highly valued gemstone. It is generally found at 6 to 18 m below ground level in the deeply weathered claystone layer of the Griman Creek Formation of Early Cretaceous age. This forms a distinct layer below the overlying sandstone and conglomerate of Tertiary age.
Opals at Lightning Ridge are found in two forms: rounded nodules, termed 'nobbies'; or in seams. The opal bearing material is a soft greyish claystone often referred to as 'opal dirt'. Opal is generally extracted by underground mining and a typical mining operation involves sinking a vertical shaft and driving horizontal shafts, or 'levels', to obtain opal dirt. Some open cut mining is also undertaken in the area.
In areas where opals are found as 'nobbies', opal dirt is brought to the surface where it is transported by truck to a puddling site for processing. Methods for processing the opal dirt typically include wet (or sometimes dry) puddling techniques usually by mechanical means. This puddling breaks up the claystone, the finer portion of which is then discarded, leaving rock materials which are further examined and hand sorted to identify individual nobbies.
In areas where opals are found in seams, gemstones are usually recovered at the working face of the mine.
Mining generally occurs along defined low ridgelines (so called 'ridge country') where the opal bearing material is closer to the surface. There are more than 200 distinct opal fields that occur on the 'ridge country'. These opal fields may be isolated or occur in groups. The main opal field "groups" located at Lightning Ridge are; Coocoran, Grawin/Carters, Glengarry/Sheepyards, Wyoming, Jag Hill and Mehi.
Opal was first discovered in the Lightning Ridge area by European settlers in the 1880s, but its potential was not realised at that time.
Although some confusion exists, the sinking of the first shaft at Lightning Ridge is attributed to boundary rider Jack Murray in 1901 or 1902, while the sale of the first parcel of opal from the field was made by Charlie Nettleton in 1903. Nettleton's first attempt to sell opal from Lightning Ridge met with failure when he sent a parcel to a firm of Sydney jewellers, who assessed the stones as practically worthless, offering ten shillings for the lot. Nettleton and his mate then took their opal parcel of about 3.5 kg to the already established While Cliffs field, where they sold it for £15. It was a lot better than ten shillings, but still a fraction of what the opal would be worth a few years later.
At first the strange dark opal from the new field of Lightning Ridge was not readily accepted in the jewellery trade, and it was some years before its true value and potential was appreciated. But once established, Lightning Ridge black opal became the most desirable of all opal. Many famous big stones were won at Lightning Ridge over the years, including the Flame Queen, the Pride of Australia, the Red Admiral or Butterfly Stone, and many more.
Opal mining takes place within a Mineral Claim using methods outlined below (An extract from "Opal Mining within the Narran–Warrambool Reserve, Lightning Ridge, Review of Environmental Factors" dated April 2004, by Parsons Brinckerhoff).
Underground mining is the most common method employed in the opal fields surrounding Lightning Ridge. This method involves sinking a shaft through the sandstone and conglomerate layers, either by hand or with a one metre Caldwell drill, to reach the claystone, where 'levels' are then dug horizontally through potential opal bearing material. These 'levels' are dug either by hand or using power tools such as jackhammers or underground hydraulic digging machines.
The claystone mined from the 'levels' is brought to the surface by motorised hoist or winch where it is transferred to a truck for transport to a processing operation. Larger operators use a 'blower' that works like a large vacuum-cleaner, drawing the claystone to the surface through a series of pipes. There are in excess of 200 blowers in use at Lightning Ridge.
Mines with large excavations, called 'ballrooms', require propping to support the roof and reduce the risk of cave-ins. Timber props are the most common type of support used and are commonly constructed from Cypress Pine (Callitris glaucophylla), which is either sourced locally or from the Pilliga Scrub.
Open-cut mining involves excavating a large area with a bulldozer, cutting through thin layers of sandstone until the claystone level is reached. This kind of mining is generally restricted to areas where there has been previous significant underground working, or where subsidence has occurred. Once complete, open cut mines must be backfilled and revegetated.
Separate approval is required from NSW DPI to establish an open-cut within a mineral claim.
Two common techniques are applied to processing of opal dirt — wet puddling and dry puddling.
Wet puddling operations typically serve 10 to 20 individual mining operations. The following photograph shows the elements of a typical wet puddling operation. The typical arrangement comprises separate water supply and sediment settling dams divided by an earthen mound on which are located a series of transit mixers — the rear mixer portion of concrete trucks that have been removed from the vehicle. The opal dirt is delivered and transferred into the transit mixers and water is pumped from the water supply dam and mixed with the opal dirt. The fine portion of the material sinks to the bottom and is released into the sediment dam. The coarse fraction is transferred onto a tray at the back of the mixer and hand sorted using some additional water to remove the remaining fines.
Dry puddling employs the same principles as wet puddling, only using a mechanical action to break up the dirt, instead of water. The machinery can be driven by a small motor, or by hand. This method is generally less efficient than wet puddling and is generally used to reduce the bulk of the opal dirt before it is transported to a remote wet puddling site.
Opal Mining History in NSW
- 1873 - Robert Moore made the first record of pretty coloured stones from Lightning Ridge.
- 1877 - Mining for precious opal in igneous rocks began at Rocky Bridge Creek, a tributary of the Abercrombie River, in the central west.
- 1881 - Opal was discovered at Milparinka, near Tibooburra in the far west.
- 1884 - Opal was discovered in sedimentary rock at White Cliffs in the far west.
- 1889 - Precious opal was discovered at White Cliffs.
- 1880s or 1891 - Opal was discovered in sedimentary rock at Lightning Ridge (Wallangulla) and other localities in the area, but its commercial value is not recognised.
- 1890 - Precious opal mining began at White Cliffs (continuing to 1915 then going into decline).
- 1896 - Opal was discovered at Purnanga and Grenville-Bunker Field. These occurrences are near White Cliffs and so extend the size of that opal-bearing district.
- 1897 - Opal was discovered in igneous rock at Tooraweenah, near Coonabarabran.
- 1901 - Opal was discovered in igneous rock at Tintenbar, on the Far North Coast.
- 1901–1903 - Opal mining began at Lightning Ridge. The first shaft was put down around 1901 or 1902 by Jack Murray, a boundary rider who lived on a property nearby. Some time later, possibly a few months, a miner from Bathurst named Charlie Nettleton arrived and commenced shaft sinking. It was he who in 1903 sold the first parcel of gems from the field for $30, not a fiftieth of the price that could have been obtained five years later.
- 1905–1907 - An opal rush occurred at Sims Hill in 1905 and the next year the Wallangulla settlement began to develop. New Town was surveyed in 1907. Wallangulla then became known as Old Town.
- 1908 - The Nettleton or 3-Mile Flat settlement was established. Approximately 1200 people lived at this settlement during its peak. It was here that the miners began what was later to become the standard practice of shaping a stone before selling it by the carat. Opal mining at the Grawin-Sheepyard Field also began in the Lightning Ridge area and increased the importance of the opal fields in the district.
- 1919 - Opal mining began at Tintenbar, continuing to 1922.
- 1920 - The Newfield opal area was discovered.
- 1985 - Innovative work by the Geological Survey of New South Wales lead to more scientifically controlled exploration for opals.
- 1989 - The Coocoran opal area was discovered in the Lightning Ridge district.
- 2005–2006 - The estimated value of opal production in the state is about $30 million. New South Wales (and Australia) is a leading world producer of opals.