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.
Andamooka Rainbow Matrix Opal - Raw
If you look carefully you can just see the small flecks of opal within the concrete like surface. The next stage is to cut the piece into slabs so that you can then select the best areas to cut your stone. A diamond saw is needed for this stage. Alternately you can grind away excess material to shape your stone making sure you remove all imperfections which will affect the quality of the finished stone.
Andamooka Rainbow matrix is much more porous than the Andamooka matrix opal. Andamooka matrix opal is generally a fine grained whitish rock similarly interspersed with fine pieces of opal which are difficult to see in an untreated specimen. Treatment of matrix opal is carried out by soaking in a heated sugar solution before placing it in a heated sulphuric acid solution. The sugar penetrates the surface of the matrix and the acid carbonises the sugar turning the background matrix black and this allows the opal to stand out. Details on how to do this are set out in my post "Andamooka Matrix Opal".
First you cook it in sugar as with matrix opal and then wrap it in foil and cook it in an oven at a moderate heat until the sugar carbonizes. Do not do this with ordinary matrix opal as it will explode and render your matrix into a great number of small pieces. I know this will happen from personal experience.
It is usual to cut and shape your Andamooka Rainbow Matrix Opal before treating it although sometimes I treat it prior to cutting so I can see where the best play of colour is. I can then cut the stone to achieve the best colour play. But then I have to treat it again as the treatment only penetrates a little way into the stone depending on its porosity.
Unfortunately Andamooka Rainbow Matrix Opal will not take a polish because it is too porous. The only way to really bring out its beauty is to coat the stone with an epoxy finish. I use Aristocrat Liquid Glass and as long as I am very careful in getting exact proportions of the two liquids mixed in accordance with their instructions I usually end up with a satisfactory finish. You do have to ensure you get all air bubbles off the surface or it will not look right. I try and prick them with a fine brush or I am told you can use a hair drier or a gas torch to brush warm air across the top of the stone and this will break the bubbles. This material takes at least 24 hours to dry so do not touch the surface with your fingers in that time or you will end up with fingerprints on the top of the stone and have to start again.
If you mess up the coating you can soak it in Methylated spirits and then scrape off the coating with a sharp blade and then start again.
It may seem a lot of work but the end result is well worth it.
Andamooka Rainbow Matrix Opal is probably one of the cheapest forms of opal to purchase but this is beginning to change. It is only found at the Andamooka Opal Field and very little is being found there at this time. Mining has almost ceased at this field now with only a few miners using open cut methods to clear the old sites looking for any missed opal. This means most of the matrix and Rainbow matrix is mixed up with a lot of clay and dirt and is really hard to find. This means that there will be less and less on the market in the future.
This is a great pity because Andamooka Rainbow is a beautifully unique stone. It has a great play of colour and is usually cut into large stones. The photos below are stones I have cut and treated and you will see why I really like them.
40mm x30mm Andamooka Rainbow Matrix Opal
This Rainbow Matrix opal is perhaps the best I have cut. It is large and full of red and green. If it was a true black opal it would be priceless.
A 40mm x 30mm broad flash green Andamooka Rainbow Matrix opal
This stone is huge and has a broad green flash right across the opal.
A multi coloured Andamooka Rainbow Matrix opal pendant
This multi coloured stone has been made into a simple but beautiful pendant.
A batch of Andamooka Rainbow Matrix opal of various shapes
The final photo is of a batch of Andamooka Rainbow Matrix opal showing various shapes and colours. They are all very beautiful and would make lovely brooches or pendants.
Found only at Andamooka in South Australia it is unique and yet it has little value. I cannot understand why. Perhaps it is because it is "treated"? Yet the treatment does not affect the opal. It only changes the background letting the opal be seen more clearly.
I hope this post encourages you to get some of this unique gem, treat it and see the wonderful transformation from a bit of concrete to a really beautiful gemstone equal, I believe, to any other.
For more photos of Andamooka Rainbow Matrix opal, Click on Andamooka Rainbow Matrix Opal.