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.
If an opal is treated this should always be made clear to anyone purchasing the stone as the value of a treated stone is much less than an untreated black opal.
In most cases you can distinguish between the two types because the matrix opal is more porous and under a ten powered loupe you will see signs of the pores. In some cases these are not very visible and it is extremely difficult to see the difference.
There are a number of methods of treating the stones to change the colour of the background stone. The one I will describe here I have used successfully in many cases. A more detailed description of the treatment process can be found at Australian Opal Mines.
The method used is to cook the stones in a sugar syrup mixture and then in a 90% sulphuric acid solution. The sugar penetrates the pores of the limestone and the acid then reacts with the sugar to turn it into carbon thus darkening the stone. As each stone is different with the limestone having various degrees of porosity, the colour of the changed stone can vary, Some turn very black, some grey and some brown. Some stones that are very dense may have to be treated many times to get the desired colour and some are so dense there is very little penetration of the sugar solution and hence no change in colour. It is a matter keep on trying and seeing what the result is. It is generally well worth the effort.
The equipment I use is very basic. For the heating I use an old electric frypan with a thermostat control. It is very important not to overheat the opal as it can fracture when too hot. A mixture of 1 cup of sugar (white or brown doesn't seem to make any difference) is placed in a pyrex or other heat resistant bowl with an equivalent amount of water and heated until the sugar is dissolved. I usually set the frypan to about mid-range for the heating process. The pyrex bowl can be placed directly onto the frypan. Water is not needed.
Before treating the matrix opal it needs to be cut and shaped and sanded but not polished. The opal should be dried thoroughly by light heating to remove any moisture from the fine pores in the stone. Avoid touching the stone with your fingers as oil from your fingers can affect the treatment. The stone or stones are then placed in the sugar syrup solution and cooked for 8 hours plus. The time of cooking is a bit of a guess and depends on the porosity of the stones. Very porous matrix will absorb the sugar solution much more quickly than dense matrix. I do not know how to assess how long it will take but 8 hours seems a good start. After cooking let them cool down. It may be necessary during this stage to add some more water to the sugar solution if it becomes too thick due to evaporation due to the cooking process.
THE NEXT STAGE IS A VERY DANGEROUS ONE AS IT INVOLVES THE USE OF HIGHLY CONCENTRATED SULPHURIC ACID. THIS IS VERY CORROSIVE AND THE FUMES CAN ALSO BE DANGEROUS.
I use 93% sulphuric acid obtained from a chemical supplier. I place a small quantity in a pyrex bowl and place it in the frypan . The frypan is in a well ventllated shed. When putting the acid into the bowl it is recommended to use rubber gloves and eye protection as the acid will badly burn you if it splashes onto your skin, so do be careful. Also never pour the acid into water as it will react violently and could splash you.
The stones are then carefully removed from the sugar solution with plastic tweezers (not metal ones as they will react with the acid) and then carefully placed in the acid after letting excess sugar solution drip off the stones. Don't attempt to wipe the stones at this stage.
The stones can then be cooked in the warmed acid for two hours plus. At this stage they will have changed colour. They can then be removed carefully with plastic tweezers and should be placed in another bowl with a weak sugar solution or a solion of bicarbonate of soda. This is to neutralize the acid.
After a few minutes they can then be admired although there is still the final polishing to be carried out. While wet you will see the amazing transformation that has taken place. The colors of the opal will now be clearly showing.
If the matrix was quite dense you should now be able to polish the stone. It is better not to use cerium oxide or tin oxide as the polishing compound can get into the pores and be seen as tiny white spots which can mar the surface. I prefer to use a leather lap with 50,000 grade diamond for the polishing. Care needs to be taken not to apply too much pressure as this will generate a lot of heat and could crack the stone.
The more porous matrix may not polish at all. If this is the case the surface can be covered with liquid glass to produce a highly polished surface. I will discuss this treatment in a different post.
Don't forget to dispose of the acid carefully. I add the acid to water very slowly and then bury it in the garden when it is very dilute. Do not add water to the acid as it willl splutter and could easily splash onto you causing harm. If it does, liberally wash the affected area with water.
If anyone has any comments on this treatment or any alternative treatments please feel free to comment. This works for me and I have some very beautiful stones which look very much like quality black opal and have cost very little in comparison.