How to treat Andamooka matrix - with sugar and acid

If you would like to try this technique, do so at your own risk!  This blog is provided for interest purposes only in response to multiple requests for information on how we treat Andamooka and our Opalton matrix.

Finished Andamooka Pendant in Sterling Silver

This technique uses sulphuric acid.  We have another process that is acid free (check out our blog on that process – How to treat Andamooka matrix - with sugar, no acid)

Pros = Makes some beautiful opal, fun to see the colour change.

Cons = Dangers with working with sulphuric acid not to mention it looks like you have set up a meth lab in your workshop ;)  If at any point your skin comes into contact with the acid, wash immediately under running water and seek medical attention if required.


 Rough matrix about to be sliced into slabs

For the heating, we use a laboratory hot plate which has a good temperature control. It is very important not to overheat the opal as they can fracture if they get too hot.

Put 1 cup of raw sugar in a Pyrex container, beaker or other heat resistant bowl with an equivalent amount of water and heat until the sugar is dissolved. Set the temperature to about mid-range for the heating process. The beaker can be placed directly onto the hot plate.


 Variety of raw and first cooked slabs of martix

Before treating the matrix opal, it needs to be cut and shaped and sanded but not polished. The opal should be dried thoroughly by lightly heating or air dry 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 stones are then placed in the sugar syrup solution and cooked for around 8 hours (give or take). The cooking time is a bit of a guess and depends on the porosity of the stones and what colour tone you are after. Very porous matrix will absorb the sugar solution more quickly than dense matrix. It may be necessary during this stage to add some more water to the sugar solution if it becomes too thick due to evaporation during the cooking process. After the cooking time has elapsed, let the stones cool down in the sugar solution. Avoid touching them with your fingers.


 Matrix cooking in the sugar solution



Make sure you are using heavy duty chemical resistant gloves, safety glasses and a mask/respirator to stop any fumes. Use 93% sulphuric acid, which can be obtained from a chemical supplier. Make sure you are in a well-ventilated area, you do not want to be breathing in any fumes. When putting the acid into the beaker avoid splashes and spills as the acid will badly burn you if it contacts your skin.  Please be careful.  Never pour sulphuric acid into water as it can react violently and splash you.

Place a small quantity of the acid in a beaker or Pyrex bowl and place it on the hotplate. Warm up on a low temperature setting.  The stones can then be carefully removed from the sugar solution with plastic tweezers (not metal ones as they will react with the acid) allowing excess sugar solution to drip off.  Don’t attempt to wipe the stones at this stage. Gently place the stones in the acid.  The stones can then be cooked in the warmed acid for a couple of hours. At this stage should have changed colour.  While the stones are cooking prepare a bowl of bicarbonate of soda and water, a couple of tablespoons of bicarb should be sufficient.  The stones can then be carefully removed from the acid with plastic tweezers and placed into the bicarbonate solution.  This step is to neutralise the acid remaining on the stones. It may fizz and spit a bit when they first go in, so again be careful!

After a few minutes, the stones can be removed from the bicarbonate solution to check out the results. While wet, you will see the amazing transformation that has taken place. The colours of the opal should now be clearly showing. The entire process can be repeated if you would like a darker appearance.


 Matrix finished cooking ready to be shaped and finished off.

If the matrix was quite hard with small pores, 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.  This can make the finish look a little pale. Use a leather lap with 50,000 grade diamond paste for the polishing.

Don’t push too hard when polishing as this will generate a lot of heat and can crack the stone. Take your time. The more porous matrix may not polish at all. If this is the case the surface may have to be coated with a liquid glass to produce the appearance of a highly-polished surface.


 Matrix ready for polishing

Don’t forget to dispose of the acid carefully and responsibly. Add the acid to a strong mix of bicarbonate of soda and water very slowly as this should neutralise the acid.

If you do spill some acid on yourself wash it immediately with lots of running water and seek medical attention if need be.

I have tweaked this process with various temperatures and cooking times and it seems to work well.  You may need to modify a few things to suit your own requirements and the different stones being treated. Either way at the end of this process you should end up with some beautiful stones which are comparable to natural black opal.

Check out our rough Andamooka matrix opal in our online shop as well as our range of finished hand crafted jewellery   Stocks vary throughout the year.

Thanks for dropping in!

Cheers Mark


  • Robert lewis

    What oven temp did you use to treat opal with the sugar method only and the acid method please thank you in advance ?

  • lynn lumby

    very informational i live in tassie but have been able to secure some andamooka rock and i have now a machine i can cut and polish with and your informations contained here where the start that i needed to get to the cutting stage thank you ill send you some pic when i figure out how to cook it properly as there some nice fires in the one im working on now cheers lynn

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