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.
When the concentration of opal in the matrix is low, of course, there is little reason to mine it. However, larger boulders must be still broken in order to discover small patches. This process greatly adds to the cost of mining matrix opal.
The concentration of opal in the matrix may get so high that treatment is useless and the material is technically solid opal. Stones cut from this material show rich swirling colors but do not have depth, and are slightly translucent.
Another factor affecting the treatment process, is the concentration of dolomite in the matrix. Dolomite is simply limestone which is more dissolved and therefore lacks the permeability necessary for the treatment process to succeed.
Only when the granularity of the matrix and the concentration of opal is balanced just right, will the matrix accept the treatment process.
Inclusions are another element which must be considered when cutting and treating Andamooka matrix opal. As a sedimentary rock, limestone contains various inclusions of softer and harder pebbles, fossilized plant and animal remnants, and sometimes, pockets of solid opal.
Depending on the hardness of the inclusion, it may darken differently from the surrounding matrix when treated. Some inclusions are too hard, and remain light colored or appear 'dirty'. Softer inclusions may darken beyond that of the matrix.
Often the cutter can take advantage of these inclusions and create 'picture stones' which although often obscure, sometimes are startlingly depictive. More often however, these inclusions reduce the appeal and value of the cut stones.
Another problem for the cutter is that of orientation or 'facing' the stone. Andamooka matrix is treacherous in that the opal follows natural flows and changes in the composition of the limestone and because the stone is opaque, it is more difficult to predict the direction that the fire will take when shaped and then treated.
The most precious and rare of the Andamooka matrix opal occurs as a careful balance of a soft dolomitic limestone with a medium concentration of opal which has formed in vertically hanging sheets within the matrix. Generally these can be identified readily, even on the field, where larger flakes of fire can be seen in one orientation or another.
Regarding the value of Andamooka matrix opal:
First and foremost is the intrinsic beauty of the final gems. Andamooka matrix exhibits the exact same deep rich colors against a deep black background as does its far more expensive Lightning Ridge cousin.
Next, and probably most importantly, the rarity of the material. Andamooka is a small township north and west of Adalaide along the Stuart highway in South Australia. The precious stones field established around the town is only 5 Kilometers by 13 Kilometers, and being a very small town, (about 300 people), the hardships of life there are very trying.
There are very few opal miners at Andamooka, and even few look specifically for matrix opal. Andamooka primarily is known for a very fine solid crystal opal which exceeds the beauty of that of the Coober Pedy fields to the northwest. This material sells for ten times more than does matrix.
When one considers the amount of opal in the limestone, it is frequently greater by mass in ratio to the matrix, than in boulder opal or other matrix opals.
The fact that it must be treated seems to be the one factor that is limiting the current market perception of Andamooka matrix opal.
The treatment of gems has been practiced since prehistoric times, usually to make them appear to be some more valuable stone. Indeed, when Andamooka matrix is treated, the result emulates the best Lightning Ridge solid black opal. The difference however is that we are simply providing a background for the opal that is already in the stone. The process does not alter the opal, or add opal, or any other brightening or clarifying agent.
It is the opinion of many opal dealers that Andamooka matrix should enjoy a market price which is equivalent to that of other matrix or boulder opal, in that it should be somewhere between 10 and 20 percent of that of solid black opal.
Regarding the health and stability of Andamooka matrix opal:
The treatment penetrates only about one to two millimeters (maybe less) into the material. Therefore, if the stone is gouged deeply, the white base material will be visible. Within reason, the stone can be re-cut, polished, and treated with good success. Should the stone ever be cracked, it can be repaired with greater success than solid opal due to the fact that it is opaque.
The treatment is permanent and care is identical to that of solid opal with one exception. Solid opal will not be affected by detergents or bleach, but exposure to these chemicals without rinsing may cause a light 'haze' to develop on the surface of these stones. If this occurs, it can be corrected by re-polishing, or more simply, by a touch of a light body oil.
As with solid opal, various qualities of this material exist. And similarly, the highest qualities are the rarest, possessing brilliance and richness of colors which are the equal to all but the finest 'solid' black opals.
Buy finished stones or plan to cut and treat the rough yourself. Don't buy rough and then have someone else treat it. This may be the only choice you have, but you will be missing out on one of the most interesting, surprising, and satisfying experiences in your life !
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Last modified: July 8, 2007