Discovery of the Andamooka Opal Field
In 1872 a pastoral lease was taken out for Andamooka Station and 58 years later in 1930 two boundary riders; Sam Brookes and Roy Shepperd found opal at the site now known as Treloars Hill. The story goes that they were caught in a thunderstorm and whilst sheltering under a tree a pretty coloured rock caught their eye. The rock was taken back and later identified as being opal. They attempted to keep the find a secret but the word did get out triggering a rush of miners heading for Andamooka in the early 1930’s. (photo – floater)
Andamooka Precious Stones Field was proclaimed in xxxx and at the time claims measured xxx
It was soon discovered that Andamooka ground was stable and comparatively safe to mine and that Andamooka opal was of very high quality.
As a consequence Andamooka settlement grew from the first settlers in 1933 to a population peaking at around 2000 over the 1960’s with around xxx claims being worked at that time.
The topography of the Andamooka opal field is highly typical of ‘opal country’, mostly low-lying gibber plain with shallow dips and basins which can be sandy in appearance and worn ridges which indicate that water has at one time been present in significant quantities resulting in significant deposits of clay and sandstone. (pdf – opal field topographic map)
Opal is found in the level of ground resulting from the cretaceous period when the area was an inland sea alive with ancient creatures whose shells and bones are often discovered as opalised fossils. This generally lies between 4m to 30m below the natural level of the topsoil. (photo – fossil)
The Andamooka Precious Stones Field contain geological variations of quartzite, shale, sandy beds, conglomerate and fossiliferous dolomitic limestone with Adelaidean Rocks. These variations can be clearly seen in the ‘cuts’ around the opal fields with distinct ‘levels’ containing rocks at varying intervals. (photo of wall of cut)
Opal is generally found in the 2nd and 3rd levels, but has also been found in what is known as the ‘false level’ as well as the first and fourth levels. Alluvial ground can also contain opal and ‘floaters’ that lie on the surface have been forced up by movement of the ground over centuries.
Slips can be found in the levels resulting from ancient geological events and it is these that opal miners seek and follow as they are considered to be indicators of opal bearing ground.
Andamooka has a reputation for opal being present in all types of unexpected settings whether it be embedded in a block of common crystal or hidden inside a plain looking rock, or simply lying on the surface, so miners here need to be checking all options in their quest to find this elusive gemstone.
Types and value of Opal
No single opal is the same. Each stone is individual and although Andamooka has more variation in the type of opal found, these can be categorised into several basic variations.
The valuable highest grade of what is known as gem or precious opal found at Andamooka contains brilliant colours and is considered to be of the highest international standard. The lowest grade, called potch, is almost worthless.
Clear crystal opal displaying full colour variations (ROYGBIV) and good transparency is the best grade of opal which can be worth up to xxx per carat once cut and polished. This type becomes more valuable depending on the nature of the pattern. Harlequin is considered to be the most beautiful and most valuable. (pic) Occasionally valuable black opal is found on the field but this is a rare occurrence. Best quality opal jewellery can sell for many thousands of dollars.
Crystal opal has a wide variation of colour combinations ranging from less valuable vibrant blue violets and blue greens, to the next level of value containing golds, oranges and pinks, progressing to others of greater value which include distinct reds and a full range of colour.
Many customers purchasing opal jewellery have a preference for the less valuable blue green variety demonstrating that value is in the eye of the beholder. So with so many colour combinations to choose from there is something to suit most tastes and pockets. (pic of variations)
Jelly opal is highly translucent with flashes of colour play which are best seen against a dark background. For this reason jelly opal is often set with a black backing to bring out the elusive colours, but sometimes they are set without backing so that the colour plays are teased out of the environment around them. There are mixed opinions on the value of jelly opal. (pic)
Milky or what is sometimes called ‘white’ opal may contain the same colour combinations in varying degrees as seen in crystal opal, but the base-rock is milky white, which in itself varies in density. The greater the density of white and the less colours present, the less valuable the stone. However some opal jewellery customers prefer the milky variety to the crystal. (pic)
Matrix or Boulder Opal is formed from sedimentary rock. Andamooka matrix (pronounced maytrix) is a unique brown sedimentary rock. Best quality matrix contains myriads of tiny deposits of gem opal and can be cut, polished then treated to produce an attractive and sought after stone that reflects the rainbow colours of crystal opal in a dense black to earthy brown base-rock. Matrix is often used to make attractive jewellery or for items such as desk ornaments.
Potch or common opal has no play of colour within the stone. Potch can present as clear with a weak single colour ranging from violet blue to yellow or in some cases green, or it may be opaque varying from black to milky white.
Famous Andamooka Opals
The Queens Opal
References: Information in this section has been compiled with the assistance of the following publications which you may find of great interest:
- Rediscover Opals in Australia, Stephen Aracic, Kingswood Press, 1999.
- Opal South Australia’s Gemstone, Department of Mines and Energy, Director of State Print, 1992.
Mining Andamooka Opal
Opal miners must secure a Precious Stones Prospecting Permit before undertaking exploration to stake a claim. They are restricted to the area within the boundary of the Precious Stones Field and the area is also subject to the terms of an Indigenous Land Use Agreement (ILUA).
Miners look for ‘indicators’ and each has their preferred combination of these, before looking to peg a claim.
Pegging a claim
The next stage is to measure out a claim according to size with a choice of standard, large or extra large to choose from. Measurements are calculated from the nearest Mines Department survey peg. Once measured and way points recorded, the miner fills in and registers an application which is submitted for approval. If there are no issues with infringing on other claims or with the ILUA the claim is registered. The miner then erects marker pegs on each corner of the claim indicating the boundaries of the claim. The miner receives a set of ‘plates’ which must be attached and clearly displayed on each of the marker pegs. Once installed, the miner can begin operations
Methods of extraction
Andamooka ground is basically stable and relatively safe to mine, so local miners work their claims using a range of equipment and approaches.
Early methods of extraction were extremely hard work under harsh conditions using hand tools. Miners tunnelled underground working outwards from a hand dug shaft. These shallow tunnels were barely large enough to turn around in and they used hand winches to bring the dirt to the surface where it was later sorted using hand sifters. (pics)
Later extraction methods became more mechanised and a variety of methods were employed both underground and above ground using equipment ranging from jackhammers and mechanical unloaders to early model bulldozers, excavators and trucks and a range of innovations for dry noodling. Historic Machinery Display
Nowadays methods have become more sophisticated and machinery more modern, but the innovations still cause visitors to ponder who and how these amazingly effective inventions where thought of and more over constructed.
Some underground miners still employ pheumatic drills and mechanised bucket arrangements to transport the soil above ground.
Others use tunnelling machines or bobcats equipped with spikes to break through the walls, and small dump trucks called ‘dumpies’ to cart the dirt above ground.
Above ground work may be worked using a backhoe/front end loaders but the more serious operations usually use bulldozers dig the cuts and excavators to work through the level ground to load large dump trucks which cart the dirt above ground where the dirt is then sorted using a front end loader.
To extract the opal from the raw dirt, most of the larger mining operations use what is called a noodling machine and there are many varied designs to these contraptions.
The principal is that the raw dirt is loaded into a bin at which time the rocks are taken out by a grid spanning across the top of the bin, these are set aside to be examined later. The dirt is then moved via a conveyer belt into a revolving mesh trammel where the fines drop out and the ‘lumps’ are broken up and rocks chipped which theoretically exposes the opal as it travels the length of this section. From there the broken up dirt is dropped onto another conveyor belt where it enters into a darkroom to travel under ‘black lights’ that use ultraviolet fluorescence to detect opal which show up as bright white due to the water content within the stone. (pic)
The opal at this stage is called ‘rough opal’ and the next stage is to examine for quantity and quality and to clean off excess so that the opal can be seen by prospective buyers. Most precious stone dealers prefer to buy opal ‘in the rough’.
Other stones are prepared to be cut to shape and polished ready to set into jewellery and some are sold before being set, others sold in jewellery pieces.
Cutting & Polishing
There is a true art to cutting and polishing opal. The nature of the stone means that each usually has a ‘good side’. So working the stone to accentuate its best qualities is an acquired skill and good cutters can achieve amazing results.
Demonstrations of cutting and polishing can be seen at the Andamooka Underground Opal Museum located underneath the Post Office. There is an amazing display of opal from all over the world together with photographs and mining paraphernalia on show and a huge range of opals to browse. Visitors are encouraged to arrange a time ahead of their visit. The museum also offers a cutting service.