Treatments
Carbonising
Carbonising of matrix opal involves any process or method of emplacing graphitic carbon particles into the pores or spaces particularly of “matrix” type opals in order to enhance the material to a “black” body colour and hence imitate “black opal” It is revealed by microscope observation as areas of black “spots” or patches in the opal material.
Acid and Sugar
This is the most common form of opal treatment and is usually applied to Andamooka matrix opal. The process is fairly simple. The matrix opal is cut and polished to shape and then “cooked” in a saturated sugar solution. After a period of time the opal is removed and immersed into a concentrated sulphuric acid solution. The reaction of the acid and sugar places carbon as black spots into the porous surface layer of the matrix opal turning it black and hence imitation black opal.
Charcoal burning and Smoke Treatment
This form of opal treatment has been recorded in history. It is said the matrix opal can be wrapped in newspaper or bark or similar and placed into a fire, carbonising the material and depositing the carbon in the matrix opal. More recently this form of treatment has been applied to the more porous varieties of Ethiopian “hydrophane” opal
Fracture Filling and oiling
Opal that has crazed is treated by attempting to fill the crazing cracks with an “opticon” or similar resin material. Also known as a treatment (for durability) is the grinding out of sand spots in the black potch of the gemstone and filling the hole left with resin, even though this procedure may never reach the colour bar and be seen face up in the opal.
‘Smoke’ treated Ethiopian Hydrophane opal.
A substantial crazed crystal opal. Fracture filling and
“oiling” attempts to hide these lines however in most instances the attempts fail and the crazing is still evident with magnification as low as 10x.
Resin Treatment
Rein treatment of Queensland boulder opal has been used recently. Some Queensland boulder opals are treated in this fashion. The issue is that the opal which is normally naturally attached to its ironstone host sometimes does not completely adhere to the ironstone, possibly because of a layer of sand or silt. The treatment infills the join between the opal layer and the ironstone backing and the treatment is very difficult to observe without sophisticated analytical equipment, however the treatment is very limited in its application.
Surface Coating
Recently it has been determined that some specimens of Queensland boulder opal have been treated by the deposition of a layer of Aluminium oxide (Al2O3) in order to overcome the softness of some opal. Opal has a Mohs scale hardness of 5-6 whereas Aluminium Oxide has a hardness of 9. This treatment is very limited at the present time, however if it becomes prevalent it will be difficult to detect without suitable analytical equipment.
Resin treated boulder opal.
A surface coated boulder opal.