The term matrix opal refers to a type of opal where there is a base stone with tiny pieces of brilliant opal interspersed within this base stone. The base can be limestone, sandstone or even ironstone. At Andamooka and apparently nowhere else in the world, two types of matrix opal are found. One is called Andamooka Matrix and the other is now called Rainbow Opal Matrix.It used to be called "concrete" as this is exactly what it looks like. A limestone and sandstone matrix is found in Queensland and is called Boulder Opal.
In 2012, my wife Barbara and I went on a trip into central Australia to escape the wet and cold of Melbourne. As with most of our trips we tend to get diverted into at least one opal field. On this trip I was lucky enough to get to visit three of the fields namely, Coober Pedy, Mintabie and Andamooka. I have described the Mintabie trip on another Post so this one is about our Andamooka Opal Field trip. We have been here several times before and always enjoy the visit.
There are quite a few theories on how opal was formed and I will try and give a summary of the most popular theories in as simple language as I can. I am not a chemist, nor a geologist, so I may get some details wrong but I hope you get the gist of the process.
Opal is basically made from silica with a water content varying between 6% and 10%. For those chemists among you the formula is SiO2nH2O. Silica is a very common element and we know it as sand or quartz. The hardness of opal is between 5.5 and 6.5 on the Mohrs scale of hardness which makes it a relatively soft material to cut compared to saphires, topaz etc.
We have driven on the Stuart Highway many times but have always sailed past the turn off to Mintabie just a few kilometres past Marla. This time we were determined to visit Mintabie as it is only about 35 km off the Stuart Highway.
We camped at the Marla Road Station Caravan Park, which by the way, is a lovely grassed park with all amenities including power. Leaving our van here we set off quite early to Mintabie. The road in was a little corrugated but easily accessible in the dry by most vehicles.
In 1926, Minnie Berrington, a young typist from London arrived at Coober Pedy becoming one of only 3 women on that opal field at that time. Conditions may not look all that good at Coober Pedy today but at that time they would have been much worse.
Access to the town, if you could call it a town then, was appalling. The road from Port Augusta was unmade and was really just a dirt track. Supplies were difficult to obtain and the weather was extremely hot in summer with little shelter and certainly none of the "mod cons" we have today such as airconditioning and refrigeration. Food and water was even difficult to come by. This did not deter her and for a while she was employed to look after Jacob Santings store at Coober pedy.
In the early 1930's there were a few miners working at Andamooka. One of these miners was an old European who lived in an old dugout away from the main centre. He was regularly seen walking across the hill to collect water from the well in the main centre, but he rarely spoke to anyone. At that time there was a tin store near where the school is now, run by Reg Absalom. During a long heatwave it was realised that no-one had seen this miner for several days, which was quite unusual. Reg went looking for him and found him dead on the hill. He'd been dead for about three days. The decision was made to bury him where they found him, and this was the start of the original cemetery.
In my teens I had a strong interest in fossicking for gemstones. This led to gold panning and gemstone fossicking trips to Beechworth, Walhalla and other similar places,
In 1966 my cousin Ken and I decided we should try and find some opal as we had read about its beauty and the possibility of finding chips and small pieces rejected or missed by the miners at some of the older fields. After some study we settled on an old field called "Pride of the Hills" situated in Queensland west of Cunamulla and east of Toompine. We had no idea of what to expect but headed off in my Cortina sedan in September of that year. We took plenty of fuel, water and food to last the two weeks as well as picks, shovels and sieves.
Andamooka is situated approximately 360 km from Port Augusta. It can be reached by turning off the Stuart Highway at Pimba and travelling along sealed roads via Woomera and Roxby Downs.
Andamooka was one of the later opal fields found in Australia.Opal was first found on the Andamooka Station in 1930. As in all opal discoveries there are a number of claims as to who first discovered opal in the area but the most commonly accepted story is that Roy Shepherd and Sam Brooks, who were working at the Andamooka Station and shifting camp from one place to another, discovered opal in the area now known as Andamooka Opal Creek. They had climbed a hill to watch their horses and to fill in time were throwing rocks in a challenge to see who could throw the furthest. One of the rocks was covered in opal.
Andamooka matrix opal is basically a limestone base rock impregnated with tiny pieces of precious opal. Because the limestone is very pale it is often difficult to see the play of opal colour in the stone and hence a method of darkening the limestone background was developed so the colour of the opal could be seen.
The treatment described here in no way alters the opal component of the stone. It merely allows it to be seen with the naked eye. It is amazing how the stone can be transformed from a pale stone with seemingly little colour to a dazzlingly brilliant black opal. Purists will often oppose such treatment but if it can bring out the unseen beauty in a piece of opal I can see no reason not to do it. It also allows those of us who cannot afford good quality opal, the chance to cut brilliantly coloured stone at a very low cost as matrix opal is so much cheaper.