For the people who know me, they are also mostly likely to know that I often qualify Ethiopian opals as "the best opals ever"! Is that a biased opinion? Maybe... But for sure, some of them are just exceptional!
Opal from Wollo, Ethiopia. One of my favorites, it has everything: the play-of-color, the perfection of the structure (pseudo-monocrystal), and the digit pattern. Photo and sample: F. Mazzero, extracted from Rondeau et al. (2011).
What is an opal?
Opal is an amorphous (opal-A) to poorly crystallized (opal-CT) silica that contains water (SiO2, nH20). The water content usually varies from 4 to 10%, but in some cases, it has been reported to be only 0.8% and as high as 21%. It has a specific gravity of about 2, but again, with extreme cases reported down to 0.67 for extremely porous opals (they would actually float!) up to 2.3. Its index of refraction is in between 1.37 and 1.47 with a main value at 1.44. Hardness varies between 5.5 and 6.5, it is therefore quite a soft gemstone (softer than quartz, which has a hardness of 7).
Some opals (either opal-A or -CT) can present what we call play-of-color, which is the fiery rainbow of colors that dances around when moving the stone. This effect is due to the diffraction of light on a network of spheres (for opal-A) or lephispheres (for opal-CT). To learn more about the play-of-color of opals, and why it is, most of the time, absent from opals (the opals are then referred as common opals), you can download here the article from Gaillou et al., (2008).
White, play-of-color opal-A from Tecopa, California. The silica spheres are arranged in a nicely organized network (not the staking faults though!). This opal is very porous, due to the lack of cement in between the spheres.
White play-of-color opal-CT from Mexico (after HF etching). The network is made out of silica "lephispheres", giving also rise to the play-of-color.
Click to read more about what makes Ethiopian opals special!
What so special about Ethiopian opals?In terms of play-of-color, that is the locality where you will find the most diversity.
From this opal, with patches of colors representing the entire rainbow:
Opal from Wollo, Ethiopia. Copyright: Opalinda.
To this opal with an homogeneous play-of-color:
Opal from Wollo, Ethiopia. Photo by Mazzero, extracted from Rondeau et al., 2011.
You can even have some layered ones:
Opal from Wollo, Ethiopia. Photo by Robert Weldon, extracted from Rondeau et al., 2010.
Now, there are also some freaks out there: the perfect pseudo-crystals. While most opals have their silica spheres ordered at a small scales (giving the patches of play-of-color), some have a long distance order, achieving up to one centimeter or more. So there is no patches of play-of-color, but points displaying spectral colors.
This very unusual opal from Wollo (4.43 ct) displays a perfect diffraction pattern. Spectral colors formed small points that moved in a synchronized fashion and changed color when the light source was moved. Photo by B. Rondeau, extracted from Rondeau et al. (2010).
There are also some "rainbow" opals, where each patches of play-of-color display the entire rainbow of spectral colors. As B. Rondeau mentioned on this website, "The exact reason for these rainbows to appear remains unknown, but it could result from the diffraction on a curved network or on a striated surface". Click here to see a video of this effect.
Patches of pure spectral color forming a rainbow. Photos by B. Rondeau. Extracted from: above: Rondeau et al., 2011; below Rondeau et al., 2010 (9.51 ct stone).
In opals from both Mezezo and Wollo, "digit patterns" are sometimes visible, giving them this (almost) unique character. So far, I've seen this pattern only in some rare opals from Nevada (never as strongly defined though). It consists of a columnar structure with the columns with transparent opal presenting a play-of-color, while the walls of the columns are made of a milky or opaque, common opal.
Beautiful digit pattern exhibited by this opal from Mezezo, Ethiopia. Notice that it also behaves like a single crystal for the play-of-color. Photo: F. Mazzero.
This is the same opal as in the first picture of this post. Just stunning. The columnar is seen from above, so only cells can be seen. The inside of the cells are made of transparent play-of-color opal, while the thin walls are made of milky common opal. Video and picture taken at the Gem & Mineral show in Tucson, Opalinda & Eyaopal booth.
Icing on the cake: some opals from Wollo contain fossilized plants (stems, twigs and roots)! You can have the typical silicification, with silica filling voids of existing plants. This silicification phenomenon is diagenetic process that occurs after sediment burial.
Wollo opal that filled wood. Photo: Mazzero, extracted from Rondeau et al., 2011.
But most commonly, plant fossils are embedded in Wollo opal, suggesting that the opal formed in a silica-rich water where plants developed, that is, in a soil at the surface of the parent rock.
A plant fossil in Wollo opal (main stem about 1mm in width). Photo: F. Mazzero. From: Rondeau et al., 2012.
A stem (about 1mm wide) in Wollo opal. Photo: B. Rondeau. From: Rondeau et al., 2012.
More stems in Wollo opal. Photo: B. Rondeau. From: Rondeau et al., 2010.
Are Ethiopian opals stable?
The answer is: just as much as Australian opal. Meaning: There is always a risk that an opal with crack ("craze") or turn milky / white. A brilliant article has been recently published on the subject by Rondeau et al., 2011, in InColor. So, as for Australian opals, the best is to buy an Ethiopian opal from a trustworthy dealer. Most of the time, an opal begins to craze or become white either after being unearthed, or after cutting. It is better to keep an opal in storage for a while before selling it. I know that some Australian dealers keep their opals for 2 years in storage before selling them... And that is wise. All in all, unstable opals have been reported from worldwide localities, unfortunately.
Here is an example of a crazed Australian opal:
"Crazed" (cracked) Australian opal, viewed in transmitted light (left) and reflective light (right). Photo by F. Mazzero, from Rondeau et al., 2011.
And an example of an Ethiopian one.
"Crazed" (cracked) Ethiopian opal. Photo by F. Mazzero, from Rondeau et al., 2011.
Ethiopian opals suffer from the fact that they are hydrophane ("porous" opals, gaining weight when put in water). But as reported in Rondeau et al., 2011, there is no obvious link between unstable and hydrophane opals. However, so far, a much higher percentage of opals from Wollo (Tenel Wega, new deposit) has been recognized stable compared to opals from Mezezo (first found in Ethiopia).
Wollo (Wegel Tena, Ethiopia) opal: a resistant opal!
Here is a quote from Rondeau et al., 2010, which says it all:"We noticed by accident that Wegel Tena (Wollo) opals could sustain a fall from 1.5 m onto a concrete floor with no visible damage, even under the microscope. Repetition of this test on five oval cabochons did not produce any sign of damage. The same experiment with five oval cabochons from the Mezezo deposit and three oval cabochons of white opal from Australia (including one boulder opal) led to breakage of all samples."
And I've seen it live. Dealer F. Mazzero doesn't hesitate to throw his beads of Ethiopian opals on his glass booth at Tucson! It's worth a stop (on top of all the high quality opals he has on display)!
Also... Just to let you know: Dior and Van Cleef and Arpels now use a lot of Ethiopian opals for their creations...
Paysage d'opale (California rêverie collection, Van Cleef and Arpels): A 100.11ct opal from Ethiopia. Copyright: Van Cleef and Arpels.