Dec 07 2016Ethiopian Opal-by Gregory Kratch-The GemGuide

Dec 07 2016Ethiopian Opal-by Gregory Kratch-The GemGuide

Fig. 2. Faceted Ethiopian opal. Courtesy of Steve Moriarty, Moriarty Gem Art.

Courtesy of The GemGuide September/October 2016

For nearly a decade now, the new find of opal in Ethiopia has been both welcomed and debated on issues of stability and high water content. Some of the material can be quite beautiful and with the quantity that is now available, we need to understand how to handle it better.

The recent discoveries of opal in Ethiopia, and in particular the finds at Wegel Tena in 2008 have had a profound effect on the world opal market. Opal is now more popular than ever. With its astounding brightness, remarkable new patterns, and unusual physical properties, this opal is quite different than the Australian opal which has been the market standard until now. Unlike the most Australian opal that comes from sedimentary deposits, the new opal from Ethiopia is mainly formed in a volcanic environment. Ethiopian opal is classified as opal-CT. This type of opal is composed of lepisheres or small balls of crystals of quartz group crystals, tridymite and cristobalite, which are polymorphs of SiO2, produced under high temperature and pressure. Australian opal is classified as amorphous (opal-A).

The two opal rough specimen photographs are from Wollo. While the rough just below is
spectacular for the pattern and spectrum, the rough further down shows another side to the
field’s potential—natural dark opal form Wollo.Courtesy of Francesco Mazzero/Opalinda.

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Ethiopian opal from Wegel Tena is a hydrophane, but very different from the hydrophane opal of Australia. Hydrophane opal is porous, and can absorb water. When Australian hydrophane
opal is soaked in water, it will absorb some and show a play of color (often spectacular) that slowly dims or disappears when the stone is removed from the water and dries out. Because Australian hydrophanes only show their best play of color when wet, they are therefore not suitable for jewelry. However, the Ethiopian hydrophane opals show their best play of color when dry, becoming more transparent and less colorful when immersed in water. Once removed from the water, they will at first become very cloudy to opaque, and then gradually return to their original state over a period of a few hours to several days as they dry out. Stones should not be heated or treated chemically to speed up this process. This capacity to absorb water and release it with no damage to the stone is remarkable. Specimens I have tested have varied widely in their ability to absorb water with stones ranging from 4% to 15%, based on the dry weight. A gain of 20% was reported by one dealer. Therefore, I recommend that all opal be correctly identified at the time of sale so that the owner may properly care for it, and keep it at its best and most beautiful. The CT hydrophane opals are very different in many unique ways from what we have predominately seen in the world market to date and must be cared for accordingly.

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Stability is always a concern with any opal, especially with new sources if they have little history. Although Wegel Tena seemed to offer a new supply of stable opal, there have been incidents of instability, cracking and crazing. It is imperative to remember however, that all opal fields produce opal that is unstable. Hungary, Australia, Mexico and Brazil have all produced their fair share of unstable material, some of which unfortunately entered the market. Ethiopia is a whole new frontier. We will be better able to predict its stability, as we explore and discover this new source of gem opal. I have found that once stones were cut, very few have developed any problem. Similar experiences are shared by other experienced cutters. As the opal industry grows in Ethiopia, I am sure it will develop the same checks and balances within the industry itself of reputable miners, dealers and cutters that hopefully will help keep unsuitable material out of the trade. No one wants to sell bad opal; it is not good for your reputation or the industry. It would be prudent to buy from people who have a longstanding reputation with opal in the current market and who you can easily contact if any unexpected problems develop with your stone. Some dealers may even offer time limited guarantees of up to six months for unset stones. Discuss any concerns you may have with your vendor at the time of purchase. Everyone wants a happy customer.

This new opal from Wegel Tena is also exhibiting some characteristics that are quite different and unique. Rondeau et. al. published a detailed study of Wegel Tena opal in Gems and Gemology Summer 2010. Cut opals from Mezzezo, Ethiopia and Australia (including a piece of boulder opal) as well as Wegel Tena were dropped on a concrete surface from a height of 1.5 meters. The stones from Wegel Tena showed no damage even under the microscope. All the other samples showed breakage. Its unusual durability makes it a great candidate for faceting, creating stones with kaleidoscopic constellations of color, a whole new avenue to explore. In a different experiment by Stone Group Laboratories, selected material from Wegel Tena was subjected to smoke treatment to darken the stones, with an astonishing survival rate of 90% despite the high heat and extreme dryness. The physical structure and hydrophanous nature of the treated stones, among other things, differentiate them from natural black Australian opal. Although they are attractive, they can never rival the finest blacks of Lightning Ridge.

This remarkable new opal, is quite unlike the others in many ways. It should be treated, handled and stored differently as well. I am sure that these new fields of opal in Ethiopia have many wonderful surprises in store, and we should welcome this new imperial addition to the court of the queen of gem; opal.

“…which contains the fire of the carbuncle, the glorious purple of amethyst, and the sea green of emerald all shining together in incredible union.” Historia Naturalis, Vol 37. Pliny, 23-79 AD.

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