Depending on the origin of the opal, the durability and stability of the material is variable. Mexican, Brazilian, and Australian opals appear to be quite stable. Chocolate colored Ethiopean opals from the Mezezo deposit are quite attractive but crack easily as a result of even the slightest temperature variations.
Wegel Tena Opal
The most important and exciting new discovery of opal is also in Ethiopia, but at the deposit near Wegel Tena, 200km north of Mezezo and 550km north of Addis Abba.
Opal from this new area has been mined since 2008. The entire region around Wegel Tena consists of a volcano-sedimentary sequence with alternating layers of basalt and rhyolitic ignimbrite. Over the entire region, only one thin seam is mineralized with opal. The opal is extracted by artisanal miners using picks, hammers, and shovels.
The body color is typically white with some brown and play of color. Unlike the material from Mezezo this material is remarkably tough and resitant to crazing.
Perhaps the most unusual feature of the material is its hydrophane (water absorbing) character. When soaked in water, many of the stones become transparent. However, the phenomena is completely is reversible and repeatable and all the color will return upon drying although drying may take a few days.
The absorption of water is related to the opal's porosity and as a result, the specific gravity and weights of the gems may vary by as much as 10% upon immersion in water.
Some of the stones display pinpoint play of color as well as an interior lizard skin or digit pattern which differentiates these opals from other opals. Because of the unique characteristics, even experts have confused this material with synthetic opal.
Although a thorough geological exploration will be required to determine the extent of this deposit, it is already the most significant discovery of high quality opal in many years.