Wollo (Welo) opal from Ethiopia can be recognized as a new type of opal because of its ability to absorb water, which affects its transparency and opalescence. After soaking in water most samples of this opal become transparent and lose their color. This phenomenon is completely reversible and after drying opal fully regains its color and optical effect.
The opal from Wollo Province in Ethiopia is mostly light, translucent with play-of-color. Some of these opals are being darkened via a smoke treatment to resemble natural black opals. Black opal is the rarest in nature and most desirable. This type of treatment is stable and resistant to cleaners and solvents.
According to the results obtained by Stone Group Laboratories in their study, the smoke treated opal can be identified by the following features:
- Carbon peaks at 1150 cm-1 in Raman readings (present in most smoke treated opals tested and absent in all untreated opals);
- play of color, predominantly in the red and green colors;
- cracks and crazing (in less than ten percent of the samples) that are not commonly observed in Wollo opal;
- smoke concentration in scratches or cracks (may show);
- Carbon (smoke) spots inside smoke treated opal;
- Surface pitting on reflected surface of treated opal.
The whole article “Smoke Treatment in Wello Opal” from Bear Williams, CG and Cara Williams, FGA can read here