The answer is yes. And there are two methods, and two source materials.
Andamooka Matrix Opal & Sugar Treatment
At some point in opal ‘s history (perhaps 25-30 years ago), a discovery was made regarding an already known type of opal matrix from Andamooka in Southern Australia. The dreary looking opalized porous limestone-matrix from the small desert township could now be “caramelized” to look just like top quality black opal from Lightning Ridge, Australia.
The porous limestone matrix at the heart of this transformation consists of tens of thousands of microscopic cavities running in a “honeycomb structure.” Millions of years ago, this structure prevented opalization into the large single pieces that we normally associate with opal. What Mother Nature created instead, was thousands of baby opals aggregated together by the limestone honeycomb.
However, the white chalky limestone-matrix that binds these micro-opal s together, also smothers their potential. By obliterating the play of color or diffraction effects, the opal matrix appears washed out, rendering a worthless chalk-like substance. However, when the matrix is wet, it is a different story—a fantastic opalescence is released. Much to the frustration of Andamooka’s earlier opal miners, they could fleetingly see the matrix’s potential when wet, but they could not figure out how to prevent the valuable effects from literally evaporating before their eyes!
25-30 years ago an unknown gemologist subjected the matrix to the now well-known treatment usually applied to porous gems. The matrix was saturated in a strong sugar solution. This was then followed by treatment in a sulphuric acid bath. Just as with agate, the acid-sugar reaction deposited carbon into the pores of the matrix, predictably turning its color jet-black.
Somewhat less predictable was the effect of the carbonization upon the opalescence. The transformed matrix no longer obscured light. The opalescent colors radiated with vastly improved intensity, resplendent against a dark surface, reminiscent of the finest Lightning Ridge black opal s.
Uninterrupted flowing wisps of color streak across this Lightning Ridge Black opal.
In contrast, this Andamooka matrix opal clearly shows the “honeycomb” or “chicken-skin” effect.
- While these Andamooka matrix opals may look almost identical to top quality black opal from Lightning Ridge, a little knowledge can easily separate the two. Sometimes, these “caramelized” opals can show a slight grayish look, but the real features to examine are:
- Andamooka matrix opal always displays a slight “granular” surface effect that is not seen in regular opal. The play of color in Andamooka matrix opal follows the honeycomb and can clearly be observed under a 10x loupe. Sometimes large “interruption patches” stop the play of color, leaving dark patches with well-defined colorless edges.