Lightning Ridge opal is found in sedimentary rocks, laid down during the Cretaceous period, about 110 million years ago, when the area was under a freshwater sea. The opal-bearing material is a softer claystone, which is found in seams or veins, surrounded by harder sandstone material.
Although the process is not yet fully understood, it's thought that opal is formed by water dissolving silicate minerals as it makes its way down through sandstone. When the water collects in voids or cracks and evaporates, it leaves the silica crystals behind. Over millions of years of this process, Opal is formed. There are many different types of opal, ranging from low-quality 'potch', which is a plain grey colour, right up to the most highly prized Black Opal. In some fields, although not in Lightning Ridge, opalised fossils can be found, where the original material, for example bones or teeth, has been completely replaced by Opal deposited in the void as the acidic water dissolves the organic material. The crystalline mineral structure acts like prisms in a variety of orientations, causing refraction of light as it passes though the stone. In the higher quality stones, this creates "the fire in the stone", which is the elusive goal of every miner.
Even commercial mining is a pretty low-tech business, conducted mainly in small shafts and leads, with the veins of (hopefully opal-bearing) claystone removed by hand, or with the aid of pneumatic drills and vacuum pumps known as blowers (although they actually suck). This material is then taken out to a local dam and washed and tumbled in modified cement mixers, with the silt discarded and the harder boulders retained. These are then examined by eye and hopefully there will be some colour in each load.
The nature of the deposits meant that in most cases, mining was done by shafts and leads, mostly within 1 and 30 metres below the surface. When following a narrow vein of claystone, it's much more efficient to use this technique than to use open-cut mining, which removes tons of overburden and rock that will not hold opal. However open-cut techniques were used and some large and prosperous mines resulted.
The area around the town is rich in deposits and while Lightning Ridge was the largest and most famous, there is not much active commercial mining going on and the town is now mainly tourists and hobby miners, with the serious miners now working other fields in the district.
"The fire in the stone" created the same sort of jealousies, suspicions and madness as gold rushes and towns such as Lightning Ridge were wild and lawless places. The town is much more sedate these days, but still a pretty quirky place, as highlighted by the use of old car doors to create self-drive tours of the area.
The Cache
This cache is located at one of the biggest and richest open cuts in the area, known as Lunatic Hill. The hill got its name because it was so much higher, any shafts would have to be so much deeper, so the question was, 'what lunatic would mine up there ?".
The posted coordinates will have you at a couple of signs, where you can obtain the following information which you should email to me. Logs without accompanying emails will be deleted. If you would like a reply, please ensure you send your email address along with the message.
- From the "On a Slice through time" sign, What is the name of the formation we see a cross section through ?
- What stone forms the 'roof' of the underground mines ?
- From the "Lunatic Hill" sign, what was the year of mining, name and value of the syndicate's nobby ?
- Finally, although it's not a requirement, a photo attached to the log would be appreciated, so we can see the area throughout the year.
Once you have completed this cache, enjoy the rest of the sights of this interesting town and take the other coloured door tours. Please note that the green-door tour is regarded as dry weather only for 2WD and it is a rougher, more washed out track.