Where to look for opal, prospecting


Before doing any major work on the field, one has to get a PSPP (Precious Stones Prospecting Permit) which is available from the Department of Mines and four wooden pegs. Equipped with this, one is allowed to go out and peg a claim within the boundaries of the area of the precious stones field.

In the old days, miners threw up their hat and where it fell down they dug a shaft. Times have changed since then. Nowadays we know more about the geology of the mining fields. In search of opal the ground was explored and now there are maps with the major faults in the ground.

Over a long period of time, the earth moved and the surface was broken. These faults near the surface are called slips and slides. Of course they had an influence of the path where the silica (which formed opal) went. At a certain distance near slides, the probability to find a pocket of opal is very high. Now the big slides are already on maps, but how to find the small ones?

Just the way some people can find water when walking with wires or a pendulum, these slides are picked up by persons who are sensitive to that. Once a slide is found, a certain distance away a prospecting-drill drills a hole with a diameter of about 20 cm. Then it can be seen how deep the opal bearing levels are and with some luck even opal is brought up to the surface.

Then the miners evaluate if they want to start to mine. If yes, a Calweld-drill drills at least one hole with a diameter of about 80 cm to a depth of up to 30 metres. These shafts provide access to the future mine and can even be reamed out to a diameter of 2 metres so that bigger machinery like a tunnelling machine can be lowered down.




  How to mine

Earliest mining was done by pick and shovel where the opal was found very shallow in the ground. But later, sinking or deeper shafts became necessary. The mullock was hauled up to the surface by a hand windlass. Underground dirt was moved in buckets or drums by sliding along greased steelpipes on the floor. Gradually, more mechanised methods were introduced.

In Coober Pedy, there are no big mining companies. Because, although it can be determined by geological means where opal is most likely to occur, nobody can know if there is colour in it or not without actually going down and look. So the profit is too unsure for a big enterprise, it is pure luck.


It is common to form ‘partnerships’ between several people. Depending on what every member brings with him, for example a blower, he gets a certain percentage of the money the claim might bring. Expenses are often shared the same way, or a sponsor pays the expenses but does not work and will get his share. But, if nothing is found, there is no money back.

Now let us see how mining can be done:

As so often, it comes down to money. What type of mining can I afford?

 The cheapest way is called ‘Pillarbashing’. This means going down in old mines and go through them again, often with an ultraviolet light. When a trace is found, the pillarbashers extend the mine.

But there are two main problems: Firstly that opal is sometimes sitting in supporting pillars, hence the name of pillarbashers, and secondly lack of space. Without moving dirt up to the surface the mine simply fills up. And, of course, it is dangerous because the dried out sandstone might have cracks, especially in the ceiling. But the equipment for pillarbashing is very little: a generator, a set of ladders, a headlight and helmet as well as pick and shovel. Often, a drill and explosives are used too.

The conditions for mining in Coober Pedy are generally good. The ground is weathered rock and often some type of sandstone,(weathered bulldog shale). As long as the rooms underground are not too big and some supporting pillars are left, there is no need to support the ceiling by other means. The main problem underground is to get rid of mullock and dirt. It has to be transported to the surface with some kind of machinery. There are several machines in use:

Winch, in the past hand operated (Windlass), nowadays powered by compressed air or electric power.

Mainly used for persons going up and down a shaft, or small scale mining. Often mounted on the back of a car (Utility).

Yorke Hoist, similar to a winch, mounted on a vertical pipe which is stayed by wire from the top enabling free rotation. Mullock is hauled up in a bucket, swung away from the shaft and emptied.

Self unloader, or automatic bucket tipper. It can be operated by one miner alone from underground. The mullock bucket is strapped to a cradle and hauled to the surface along two parallel rails by a cable and a winder motor. Above, the rails are curved so that the bucket is emptied about five metres away from the shaft. At the point of tipping a switch is activated and the empty bucket returns underground.

With these machines only rather small quantities of earth can be moved. But they are inexpensive to run. Following machines have a much larger capacity:

Blower, works like a huge vacuum cleaner powered by a truck engine. The motor is mounted on the back of a truck or trailer. A fan generates a vacuum, and through a series of pipes (diameter 20-30 cm) mullock is sucked up the shaft. It is collected in a bin which opens at the bottom when it is full and the mullock falls onto the dump. Underground, the pipes can go for several metres directly where work is done. Sometimes they connect via a telescope direct to a tunnelling machine. And another advantage is the air circulation, which is very important when explosives are used.

Bucket elevator, a series of buckets on an endless chain take mullock from a sump at the base of the shaft. It is cheaper than to build and run than a blower. But the mullock has to be transported underground to the base of the shaft.

How is the work carried out underground? Well, pick and shovel are still essential and it is hard work.

It depends how much mullock can be transported to the surface.

Hand-mining is done with pick, jackpick and explosives.

Tunnelling machine, The most common type has booms on either side of a tracked machine supporting a cylindrical cutting head. The drum is raised and lowered on the booms to cut a rectangular drive. There are also circular tunnelling machines with a cutting head on a revolving boom in use. Working with tunnelling machines is nowadays less expensive than with explosives and a blower is essential to remove mullock. It is the fastest way to cut through the underground.

Open cut mining, is to make a huge cut in the earth with a bulldozer down to the opal level. Rarely done nowadays because of the inhibitive costs of bulldozers and fuel.



 Use of explosives   


In Coober Pedy, two types of explosives are used. ANFO (ammonium nitrate mixed with fuel oil) and GOMA (similar to Gelignite, contains Nitroglycerine), or lately Riogel which does not contain Nitroglycerine anymore.

Ammonium nitrate is available in 30 kg bags and is mixed with fuel oil (diesel), petrol makes it more explosive and therefore is too dangerous). Fertiliser too contents ammonium nitrate, because of that, people make jokes that in Coober Pedy blasting is done with fertiliser.

The mixing is often done at home with a cement mixer. After mixing, it still is not too dangerous to handle. The ANFO is then filled in tubes made out of old newspaper together with a string of detonating cord (cordex). This string ignites the ANFO. Cordex explodes much faster than ANFO and supplies the heat and pressure for ignition. The cordex itself is ignited with a detonator. This is actually the most dangerous item when explosives are used. Because it sets the main charge off and is also sensitive to static electricity. The detonator is put over one end of the safety fuse, which is set alight by the miner and burns with about 1cm in one second.

GOMA and Riogel is more expensive, but the handling and preparations are easier. It is supplied in sticks, in which a hole is made to put the detonator in.

One “shot” costs approx. four dollars nowadays. Over the last seven years the cost of mining with explosives has increased threefold!

Coober Pedy, for its number of population, is one of the largest user of explosives in the world. But there only few accidents. They are mainly due to poor handling. The building up of static electricity with a discharge from the human body to the detonator sets charges off. Or, the other reason for accidents are poisonous gases which remain in the mine after blasting. Good ventilation and recognition of static electricity are the most important factors for safety.




Noodling is a special term used when one is looking for opal on the surface. It is either done by hand with a sieve to separate the dust from the stones or with machinery. This is usually a shaking or revolving sieve from which the stones fall onto a conveyor belt. The belt moves the stones through a darkroom which is only illuminated by ultraviolet fluorescent light. In this light most opal shows up and can easily be separated from sandstone and other accompanying material. This is done by ‘pickers’, one or two people who are sitting in the darkroom. The whole machinery is often fed with a loader, what means that quite a big quantity of material can be moved.

And it is sort of fun spending warm summer nights out in the field, around a dump with a portable ‘blacklight’, looking for opal. As long as one is aware of all the shafts and does not go wandering around, it is not too dangerous.

Probably now the question arises why miners miss so much when working underground? It mostly depends on the miners, how careful they work and what they are looking for. For example it is easy to miss a small seam of opal when working with a tunnelling machine. Or even not worth to stop the machinery as long as the opal is not becoming bigger. “Time is money”, especially when working with expensive machinery. And in earlier times they were mainly after top quality and dismissed anything else.



  Formation of opal


In Coober Pedy, opal is found in sandstone (wheatered bulldog shale) which is usually covered with topsoil (russo beds). This sandstone is the deposit of rivers which flew into the sea that covered this area during the Cretaceous period (65 to 135 million years ago). During the tertiary period (2 to 65 million years ago) Australia rose and left parts of the inland to dry out.

In this sandstone there are brown bands which vary in thickness from a few millimetres to several centimetres. These mark the level of the groundwater millions of years ago. It is the dirt which was washed out of the sandstone and deposited where the water came to rest for a longer period of time. There are several levels and the lowest is usually the last or most recent level.

When the waterlevel went down for a long period of time, the concentrated deposits could not follow the water down through the sandstone and dried out with the surrounding sediments. Because these deposits were harder than the sandstone it caused tension and cracks appeared.

There is a major faultline going through the Coober Pedy Precious Stones Field which is called the Karari-fault. It penetrates the sandstone and goes further down through much harder ground. Along such faultlines earthquakes are more frequent, but the sandstone  between the hard ground below and the surface absorbs most shocks. Along with a big faultline there are many smaller faults, slides and slips where the earth was moved and displaced. Therefore the hardband levels which were originally horizontal were bent or displaced along faultlines.

The hardband levels formed a barrier to water coming either down from the surface or water coming up through the earth. Because of the great artesian basin, water is pressurised and will rise to the surface through cracks or faults in the earth. Sometimes the water had to travel a long way along the hardband to find a passage through. On the way it washed minerals out of the sandstone but when the passages became more narrow it also was filtered so that this silica became purer on the way. It infilled hollow spaces or concentrated in certain places. For example, when this solution came down through the sandstone it tended to concentrate where the level dipped. There is also a  theory that the water which was pressed up had a higher temperature than water coming from the surface and collected more minerals on the way.

Where this solution came to a rest, impurities were pulled down by gravity. It can be seen best where the solution filled in vertical cracks. The bottom consists of material that is held together by silica and falls easily apart. Further up we may find matrix then low grade grey opal with some colour before it goes over to more transparent clear material and sometimes even crystal opal. Often the silica was at rest for a while and hardened, then more silica came in from another inlet and another layer was formed. Which explains different bars of opal or potch in the stone. But it is still not exactly known how long this process took. Synthetic opal in laboratories takes only several months to grow.

Gypsum, which often accompanies opal seams came in after opal was formed and grew in crystals. It was able to force the surrounding stone apart and sometimes even penetrates opal.

The less impurities in the silica the clearer it is. But to form colour the spheres of the same size have to settle in a certain array. Another condition for colour is that the silica has a certain porosity, that means the cement holding the spheres together does not infill the voids. Then the light would travel through the stone without being affected by the surface of the spheres. Bright, colourful opal has very defined voids whereas Jelly-opal has many of the voids infilled.

Shells and pipes (belemnites) are often of better quality than seam-opal. The hollow space a shell left behind after being washed out by water provided less room for the silica than it would have had along a level. Therefore when the silica finally found a hollow space after travelling through fine cracks in the sandstone it had to concentrate there.