Andamooka Rainbow Matrix Opal is fast becoming popular with those who can't afford a "Real Opal". Originally it was called "concrete" because that was what it looked like although you could see small specks of opal in amongst the concrete like grains. It is a porous quartzite host rock in which opal has been deposited amongst the quartz grains. In its raw state you can hardly see the opal and for this reason the early miners discarded it as valueless. If you hunt around the mullock heaps or even on the roadsides in Andamooka you still have a chance of picking up  a piece. A photo of a typical piece of concrete is shown below.

More Andamooka Matrix Opal

During the last month I was contacted by Andre who told me he had a couple of pieces of Andamooka matrix opal he had collected some time ago. He purchased one of these from Cash Converters and they were part of a deceased estate. Andre was obviously quite astute as the Andamooka matrix opal was not treated. Unless you know what you are looking for, some untreated Andamooka matrix opal can be quite un-exciting (if there is such a word). It is usually  very pale with only a hint of colour in it. However if it is treated by heating it in a sugar solution for several hours and in a sulphuric acid solution the background turns black as the acid combines with the sugar in the pores of the stone to form carbon. This change of background allows the opal colours to really stand out. It is a really amazing transformation and the end result can be a gemstone of great beauty. Andre treated the Andamooka matrix opal with sugar and battery acid. Most people insist you need 98% sulphuric acid but these results show that this is not necessarily the case always although Andre now advises that the battery acid started boiling in his wall oven and in the end it was destroyed. Might be better to stick with the 98% acid and cook it in a well ventilated area. I use an old electric fry-pan and a pyrex dish and this works well.

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.

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.