Item Information

SKU BY0022a
Dimensions (mm) 33 x 21 x 5mm
Weight (carats) 39.22 carats
  • Rough



One place in the world where limestone matrix opal occurs, is in a small area of South Australia called Andamooka. For this reason, this material is called Andamooka matrix opal. When it is treated, it is known as Andamooka treated matrix opal and it looks like rare black opal.

This specimen is called concret and it does not get the top polish of top grade matrix as it is to porous.It can be kept as a specimen or wire wrapped.As it can stand a lot of heat it can be baked and liquid glass or resin placed on it to get a smooth finish.

Rough treated but not finished/polished

Weight 39.22 cts app

Size 33x21x5 mm app

The photos have been taken with the stone wet




Treatment of the limestone matrix involves a fairly simple process of simmering the stones in a very rich solution of sugar in water, followed by a period of simmering in a container of very concentrated sulfuric acid.

First you  start with the sulfuric acid. A source can be found easily in the phone book under the 'chemicals' section. It must be at least 95%. You cannot use battery acid.Rember sulfuric asic is a dangerous product and pre cautions must be taken.

Try and buy a smaller containers  as they are much easier to control, and the large drum requires some method of getting small amounts out without spilling.

First, you must cut and shape the stones but only polish them to a 8000 grit surface or so. You will finish the polish after the stones are cooked.

You must also make sure that the stones are absolutely dry. I choose to put the stones in a jar of methyl alcohol for at least one day before I cook. Longer is ok. Just make sure that when you take the stones out of the alcohol, don't touch them with your fingers ! The oil in your fingers will diminish the cooking process and even make smudgy differences in the final black background.

Next, mix up the sugar and water. The sugar should be 'refined' pure sugar, not brown, not 'raw'. I fill the beaker about half way, and then stir in the sugar. I find that about 8 ounces of water will absorb about the same amount of sugar.

The water should be distilled, not tap water. It is important because some water systems contain chemicals that will later react with the sulfuric acid, and diminish its effect.

I prefer to heat the water to the working temperature and then add the sugar. I can better watch the thickness of the solution as it develops into a clear syrup.

You don't want something as thick as Karo syrup, but just a little thinner. As the sugar cooks, it will lose water to evaporation (if no top), and after 24 hours of cooking, it's likely that you'll have to dig the stones out of the mess.

When the sugar is dissolved, and the temperature is at 170 degF, it's time to put the stones in.

Again, don't touch them. I take them out of the alcohol and let them 'air dry' for only a few minutes, (or use a very clean cloth). Then, one at a time, I slide them in, down the side of the beaker.

The amount of time that is required for the sugar step will vary according to the softness of the limestone matrix of the particular stones. But it does not hurt at this time to cook them much longer than might be required.

I've seen times when only 15 minutes in the sugar is enough, but sometimes it takes as much as 48 hours.

The only caveat is that you must make sure that the sugar doesn't turn to molassas and then to hard rock candy... You can add water to the sugar if you get to it early enough. Too late, and you can't even stir it.

And stir it you must ! Even if you have a few stones in the sugar, you must stir them frequently to allow the sugar to get to every pore in the surfaces.

Most of the time you can see that the stones have turned a golden color, but don't expect it. Not even all stones from the original 'mother' will turn the same color. It depends on how uniform the original matrix was.

When you decide that they are ready, you can take the beaker off of the hot plate and let it cool. You must be careful at this point that the sugar doesn't solidify. If I suspect that it's too thick and may do this, I first pour off most of the syrup to the point where it just covers the stones. Later I can spoon out the stones...

When they are completely cooled, I simply wipe them off with a clean cloth and put them into a covered box until I am ready for the 'acid'. Again, it's tempting to handle the stones with your fingers. Admire them, but always use plastic tongs.

The 'acid test'

Even if you have handled strong acid before, it's very likely that you will get burned at some point in the process. Hopefully, you will only get a little drop and it will give you good evidence that you must be more careful...

Carefully fill a very clean and DRY beaker about half full with the acid.

Put the hot plate in a place where it is not in some normal 'path', and cannot be accidently bumped. I have lots of these places, but I choose always to put a rubber mat of some sort under the hot plate just in case.

Turn on the hot plate, and put the beaker of acid on. (not onto a hot plate)

Let the acid warm up to over 120 degF, and then, with plastic tongs, carefully drop the stones into the beaker. I have a little glass 'spoon', that I can use to place them on the bottom, but I never had a problem with a stone cracking by just dropping it in carefully.

After the stones are in, you will see that the acid starts to turn a light brown from the reaction to the sugar on the surface of the stones. If you have not wiped them off well, this may cause complete darkening of the acid right away. This isn't a big thing, but it will diminish your ability to see the stones in the beaker.

Carefully raise the temperature until it is at (my preference) 170 degF. If you have a hot plate like mine, you must be careful because it will very quickly get to over 200 degF, and it is possible that the stones can crack from the expansion of the water in the matrix.

For the first hour of this process, you MUST stir the stones frequently but gently, and watch how quickly they take on their black color. You won't want to do anything else at this point... it's a fantastic experience !

How long !

With some matrix material, you may find that 5 minutes is ok, but 6 minutes is too much ! I've only found a few parcels that are this quick, but most require an hour to 24 hours.

The rate at which they change will be constant, so you can get an idea of how long it will take by watching very closely for the first hour. But in any case, you must frequently check them so that you can take them out before they become 'over done'.

And 'over done' they can certainly get ! Muddy, dark, and all but worthless. The only thing you can do at that point is to sand them down and cook them again. Yuk !

As an aside, I use my home automation system to control the hot plate and to warn me of when it is time to stir the batch. In case I go to sleep, it automatically turns off the hot plate and I may have to start the cook again. But generally,

Fully Cooked

Here's where it takes a bit of experience to understand just when the stones have achieved the darkest tone that they ever will. Some stones will get black in minutes, others will never do more than get a little darker. Usually however dark they get, the opal in the matrix will be brighter, and that's the whole point.


What I have learned to do is to 'cook' some sample pieces of a parcel before I cook any finished stones. I keep records, and remember next time I cook something from the same parcel.

Often I cook stones from different parcels at the same time, and must remember that some of them must be taken out earlier than others.

But in some cases, the opal concentration is too high, and the matrix will not accept either the sugar or the acid. The stones get a dark 'honey' color perhaps, but will never 'cook' beyond that point.

Taking them out of the acid

Whenever you take them out, you must do so very gently, and using the glass rod or tongs. If all are to be taken out at once, then I choose to pour off the acid into another beaker first.

Then I let them cool for 15 minutes or so before I dump them into another bath of cool but more diluted sugar-water. Leave them in this solution for an hour or so, and then take them out, and wipe them off with a clean cloth. Discard the cloth, it will soon disintegrate from the acid.

Admire what you've done !

A little polish and they're finished

The stones still need to be given a final polish. But here you can really screw up the stones. If you find that you did not surface them well before, you will be tempted to do so now.

But if you grind too much, you will begin to see a lighten-ing of the background color. When this happens, it most often means another trip thru the process.

The treatment only penetrates to one millimeter or less !

If you did a good job before the cook, then the surface will only need a quick polish, and it's finished.

You will find that it is almost always impossible to put the same kind of polish on a matrix opal that you can put on solid opal. I've seen some that are almost perfect, but most of the time you will not be able to get rid of the 'granular' surface effects.


Within a few hours and over a period of days, you may see the stones 'weep'.

This is normal, but some parcels weep more than others. You must keep the stones where you can watch them and periodically wipe them off. The liquid that is weeping contains a lot of acid, and will eat just about any material. After a while, this will stop. If it does not, you may put them back into a light sugar solution and let the acid weep out into that. When you can no longer see any dark wisps of color coming from the surfaces, take them out..

This weeping may slow greatly, but may still continue for several weeks or months. I prefer to keep the stones until I am sure that it has stopped. I have tried other methods of stopping the reaction, but all have resulted in some kind of 'haze' or 'dust' on the surface, which, being microscopic, is impossible to remove completely, and is always disappointing.

A little more detail:

Although most of the matrix found at Andamooka is broken into small boulders and pebbles, it is sometimes found in large slabs. When these slabs are broken, it is found that the opal is indeed rare and spotty. Therefore, not all limestone from Andamooka yields precious matrix opal.

When opal occurs in matrix, it is found in different grades; varying with the concentration of opal, the granularity of the matrix, and natural inclusions.
Next, you need some glassware. There are some plastics that will resist the acid, but it's much better just to use good Pyrex glass.

You can use good quality glass containers for storage, as long as they have glass or rubber tops. But don't use anything but Pyrex (heat-resistant) glass for the cooking container.

You'll also need some glass 'stirring rods'. Smooth, solid, and not too long for the height of the beakers.

Last, you need a good controllable 'hot plate'.

I use a commercial hot plate, combined with a very good thermometer. It has a 'mid-range' of about 170 degrees F, where it is the most accurate.


Concrete ?

Where the granularity of the matrix is coarse, the stone will not take a polish, and even with a large concentration of opal, it is forever dull unless it is wetted or kept in water. This type is called 'concrete', and mimics a type of matrix opal found in Honduras.

When the granularity of the matrix is very fine, there is very little limestone to affect, and the opal cannot be enhanced. This does not mean however that stones of this type cannot be very beautiful of their own without treatment, and even with it.

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Bid Increment $1
Starting Bid $18
Auction ID 1209531
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Starts 1st May 2022 7:05pm PDT
Ends 2nd May 2022 5:58am PDT

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