Precious opal occurs in both sedimentary and volcanic rocks. Nearly 100 per cent of Australia's opal production is derived from sedimentary opal deposits.
All the Australian opal fields are situated in arid inland areas with little relief; and the opal is found at shallow depths, usually less than 30 metres, in deeply weathered rock in which there has been considerable silica movement.
Opal occurred during the latter stages of extensive weathering of the sediments during the late Cretaceous (approximately 136 to 64 million years ago) or Tertiary (approximately 64 to 1.5 million years ago) period. Silica released during this period percolated downwards with ground water through the rock mass. The opaline silica accumulated, concentrated and was subsequently deposited as a gel in sites provided by various structures. Gradual loss of water from the silica gel resulted in the hardening of the material and the formation of opal.
There have been many methods of mining developed to unearth opal from the Australian outback. Following are a few of the most common methods.
Sinking A Shaft
Although this is one of the most effective ways of finding opal, it is also the most laborious. The length of the shaft could be as short as three metres or as long as 20 metres. A drilling rig with a 36-inch bucket auger would save a miner the several days it could take several days to reach the opal level by hand.
A variety of miner's tools are needed, including a hand windlass or motorised winch that is placed over the hole to lift dirt to the surface; or an expensive vacuum-cleaner apparatus, called a 'blower'. The working pattern changes once the 'bottom' of the shaft has been reach, that is, where the opal-bearing dirt begins. Now the miner begins gouging away very slowly and carefully, forming a horizontal tunnel in the hope of finding a tiny seam of precious opal, or scattered 'nobbies' as at Lightning Ridge.
This technique is used particularly at Lightning Ridge. Once the opal dirt has been transported from the shaft by trucks, puddling is performed at specially constructed dams.
A puddler is a large mesh-lined drum attached to a motor. This device rotates and turns the clay into sludge as water is pumped into the drum. The sludge escapes through the mesh. Only the hard pieces - rocks, stones and 'opal nobbies'- are captured in the mesh. A related technique is dry rumbling.
Large amounts of opal dirt are sifted through in a short time by using a mesh tray.
Open-cut Mining
This mining technique is created by running over a large area with a bulldozer, slicing through thin layers of sandstone until the opal level is reached. Although this method is more expensive than shaft mining, the chances of finding opal are increased because such a large area is being covered.
Noodling
In simple terms, a noodler is a person who goes over what other miners have discarded as 'dud' mullock heaps. All that is needed is a sieve and a very keen eye. An abandoned open-cut mine is another good place for a noodler, using a rake and sieve for tools.
Some have even taken to large scale machine noodling by allowing large amounts of opal dirt travel on a conveyer belt under ultra-violet light, which detects the precious opal.
Cutting and Polishing A Nobby
Opal is rarely faceted but rather cut and polished into cabochons. These may be solid stones, doublets, or triplets. In the case of boulder opal, the ironstone is left attached to the opal as a natural backing.
Generally, rough opal is cut using a diamond saw at high speed and plenty of water.The nobby is then held against a grinding wheel until the colour bar is revealed and it is ready to be mounted on a dop stick with wax. The grinding wheel has a safety tray beneath it should the stone come loose from the dop stick. The stone is then held against the grinding wheel and given a final shape, removing all final faults for a smooth face. Any scratches are removed using a sanding disc and a spray of water during the process. The stone is then given a final sanding using a second disc and a little spray of water, followed by a dry finish. Any debris is then removed from the stone before it is polished. Before the final polish, the back of the stone is reshaped and finished.