by Benjamin Rondeau, Emmanuel Fritsch, Francesco Mazzero, Jean-Pierre Gauthier,
Bénédicte Cenki-Tok, Eyassu Bekele, and Eloïse Gaillou
from GEMS & GEMOLOGY, Vol. 46, No. 2, pp. 90–105.
© 2010 Gemological Institute of America, Summer 2010
Published on JTV.com: November 2011
See the end of this downloadable PDF for About the Authors, Acknowledgments, and a complete list of References.
A new opal deposit was discovered in 2008 near the village of Wegel Tena, in volcanic rocks of Ethiopia’s Wollo Province. Unlike previous Ethiopian opals, the new material is mostly white, with some brown opal, fire opal, and colorless “crystal” opal. Some of it resembles Australian and Brazilian sedimentary opals, with play-of-color that is often very vivid. However, its properties are consistent with those of opal-CT and most volcanic opals. Inclusions consist of pyrite, bariummanganese oxides, and native carbon. Some samples show “digit patterns”: interpenetrating playof- color and common opal, resembling fingers. The opaque-to-translucent Wegel Tena opals become transparent when soaked in water, showing a remarkable hydrophane character. White opals from this deposit contain an elevated Ba content, which has not been reported so far in opal-CT. The fire and crystal opals are prone to breakage, while the white, opaque-to-translucent opals are remarkably durable. The proportion of gem-quality material in the Wegel Tena deposit seems unusually high, and 1,500 kg have already been extracted using rudimentary mining techniques. The deposit may extend over several kilometers and could become a major source of gem-quality opal.
In early 2008, a new source of play-of-color opal was discovered by farmers near Wegel Tena in northern Ethiopia (Fritsch and Rondeau, 2009; Mazzero et al., 2009; Rondeau et al., 2009). Since January 2009, the deposit has been worked by about 200 local miners. The opals are mostly white, which is uncommon for play-of-color volcanic opal, and may resemble material from Australia or Brazil (figure 1). Some fairly large pieces have been polished (figures 2 and 3). The Wegel Tena opals differ from those found at Mezezo, in Ethiopia’s Shewa Province, or in neighboring Somalia, which are mostly orange to red to brown (e.g., Koivula et al., 1994; Gauthier et al., 2004, and references therein). Two of the authors (FM and EB) traveled to the locality on several occasions. They gathered representative gem material, as well as surrounding rocks, and discussed the opal and its extraction with the miners to develop a better understanding of this promising new deposit.
The opal mining area lies in Wollo Province (also spelled Wolo or Welo), ~550 km north of Addis Ababa and ~200 km north of the Mezezo opal deposit (figure 4). The locality has also been referred to as “Delanta,” which corresponds to a former subdivision (or “awraja”) of Wollo Province. The region containing the deposit is called Tsehay Mewcha, a large area that encompasses scattered farms and a small village, about 17 km northeast of the village of Wegel Tena (figure 4). Tsehay Mewcha is situated on a plateau at an altitude of about 3,200 m. The opal occurs in a horizontal layer that is exposed on a cliff above a canyon tributary of the Blue Nile River. This layer is ~350 m below the top of the plateau (figure 5). Tsehay Mewcha is accessible with a fourwheel- drive vehicle, and the various mine workings are reached by walking down the steep canyon for 30 minutes to more than one hour. Hazardous conditions are created by cliffs in the digging areas, as well as by falling rocks due to mining activities (see www.opalinda.com for more information on access conditions).
The entire region around Wegel Tena consists of a thick (>3,000 m) volcano-sedimentary sequence of alternating layers of basalt and rhyolitic ignimbrite. The layers of basalt or ignimbrite are a few meters to hundreds of meters thick. (Ignimbrite is a volcanic rock of andesitic-to-rhyolitic composition that forms sedimentary-like layers after the volcanic plume collapses and falls to the ground. The particles that form this rock are a heterogeneous mixture of volcanic glass, crystals, ash, and xenoliths.) This volcanic sequence was emplaced with the opening of the East African continental rift during the Oligocene epoch (Cenozoic age), about 30 million years ago (Ayalew and Yirgu, 2003; Ayalew and Gibson, 2009).
Over the entire volcanic series, only one very thin seam (<1 m thick), hosted by ignimbrite, is mineralized with opal (again, see figure 5). Common opal and play-of-color opal most often cement grains of volcanic debris (figures 6 and 7) or sometimes fill in fractures or cavities in the rock. As a result, the rough gem material often has an irregular shape. Microscopic examination of our samples revealed that, for the most part, the host rock consists of mixed altered material, including clays, common opal, and some minor iron oxy-hydroxides. Some large crystals of alkali feldspar were unaltered, while others were transformed into clays. By comparison, the ignimbrite sampled only a few meters above the opal-bearing layer was unaltered and contained abundant quartz crystals.
The opal is extracted by artisanal miners using homemade tools (figures 8 and 9), as well as picks, hammers, and shovels provided by the Ethiopian government. The mineralized layer extends for hundreds of meters along the flank of the canyon (again, see figures 5 and 8), but the excavations only penetrate 1–2 m into the mountain. Because the diggings are not supported by timbers or other means, mining is very dangerous in some extensively worked places. Tragically, at least 20 miners have died from collapsing rock. The miners are organized in cooperatives that control the distribution of the rough opal sold to gem dealers and eventually cutters in Addis Ababa. Opal production has been significant, with over 1,500 kg of rough extracted to date.
We examined hundreds of rough and polished samples to determine the typical characteristics for this locality. We selected 33 samples to document the gemological properties and note interesting inclusions and growth features, hydrophane character, matrix, and the like (table 1). Eight of these were fashioned as cabochons of various shapes and colors (4.43–18.85 ct). The other 25 samples consisted of rough pieces ranging from ~4 to 965 g (e.g., figure 7), and most contained significant portions of play-ofcolor opal. Some had considerable matrix material. Several of these were sliced or crushed for specific testing, as detailed below.
Selected samples were examined by standard gemological methods to determine their refractive index, hydrostatic specific gravity, and microscopic features (see table 1). Play-of-color was observed with the stone against a dark background (when necessary), and spot illumination placed above, behind (when appropriate), and then perpendicular to the viewing direction (the latter to observe any contra luz effect, i.e., play-of-color revealed by transmitted light). Luminescence was observed with longand short-wave UV radiation using 6-watt lamps.
We tested the hydrophane character of 14 samples by observing the specific gravity, transparency, and play-of-color before and after immersing the samples in water for a few minutes to one hour. For the same 14 stones, the following optical tests were conducted: Polarization behavior was studied using a GIA Instruments polariscope, absorption spectra were observed with an OPL prism-type spectroscope, and the Chelsea filter reaction was determined with illumination from a strong halogen lamp.
Fourier-transform Raman spectra were recorded on all samples using a Bruker RFS 100 spectrometer. Each spectrum included 1,000 scans to increase the signal-to-noise ratio, as opal is a poor Raman scatterer.
The microstructure of five samples (1071, 1073, 1074, 1076, and 1101) was investigated with a JEOL 7600 scanning electron microscope (SEM) equipped with a hot cathode/field-effect electron gun, and a Hitachi H9000-NAR transmission electron microscope (TEM) operated at 10 kV. TEM samples were obtained by crushing a small piece of each sample with a mortar and pestle. The chemical composition of eight representative samples (1071, 1073, 1074, 1076, 1078, 1100, 1111, and 1121), their inclusions, and accompanying minerals (in samples 1100, 1106, 1113, and 1122) was measured by energy-dispersive spectroscopy (EDS) with a Princeton Gamma Tech IMIX-PTS detector installed on a JEOL 5800LV SEM. We prepared two thin sections (30 μm thick) of the opal’s host rock for petrographic microscopy and analysis of the minerals by EDS.
We conducted preliminary analyses of trace-element composition by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) at the Institute of Geological Sciences in Bern. We investigated two rough samples representative of the marketable opal: one white translucent and the other zoned white translucent and orange transparent, both with play-of-color. (These are not listed in table 1 because they were examined apart from the other samples and only with the LA-ICP-MS technique.) The LA-ICP-MS system consisted of a pulsed 193 nm ArF excimer laser with an energyhomogenized beam profile coupled with an Elan DRCe quadrupole mass spectrometer. Laser parameters were set to 16 J/cm2 energy density on the sample, with a pulse duration of 15 ns and a repetition rate of 10 Hz. Pit sizes were 60 and 90 μm. The laser-ablation aerosol was carried to the ICP-MS by a mixed He-H2-Ar carrier gas. Details on the LAICP- MS measurement conditions are available in the G&G Data Depository (gia.edu/gandg).
Stability to crazing was assessed after cutting and preforming by one of us (FM) who has handled numerous Wegel Tena opals since their discovery. A few representative samples from various Wegel Tena parcels were kept at ambient temperature and humidity, and visually observed over time. As part of a proprietary fashioning process, about 50 high-quality play-of-color Wegel Tena opals were cycled between water and air (at room temperature) for one hour and then longer periods (up to several days). We tested for toughness by dropping five fashioned opals on a concrete surface from a height of ~1.5 m, to simulate dropping a stone by accident.
The standard gemological properties are summarized in tables 1 and 2 and presented below.
From observing hundreds of rough and faceted samples, we determined that most opals from Wegel Tena have a white bodycolor, while some are pale yellow and a few are darker orange (fire opal) to brownish red (again, see figures 2 and 3). Rare samples have a dark “chocolate” brown bodycolor (again, see figure 7). Some zoned samples show several layers of contrasting bodycolor and play-of-color (figure 10).
The opals range from opaque to transparent, but most are translucent. Because the material is turbid, it scatters light efficiently, creating the white bodycolor typical of this deposit. Some of the highestquality opals are translucent and display a blue scattering bodycolor (figure 11).
Among the 33 samples tested, we observed that all opaque-to-translucent samples became more transparent when immersed in water for a few minutes to one hour, depending on the thickness of the sample. This behavior is typical of hydrophane. There were several degrees of change, the most dramatic being a transformation from opaque white to transparent colorless (figure 12). During this process, play-of-color appeared to strengthen. This phenomenon was fully reversible in one to a few hours, depending on ambient humidity and the thickness of the gem.
The opals typically displayed a mosaic of pure spectral color patches against a translucent white bodycolor. Generally all spectral colors (from red to violet) were observed in the play-of-color samples, often with large patches of red and orange. The intensity of the play-of-color varied on the millimeter scale, from intense to none, even within the same sample. We did not notice any contra luz material (which is typically transparent).
Play-of-color was commonly distributed along parallel columns that resembled fingers. We refer to such features as digit patterns (figure 13). The playof- color digits were embedded within common opal of slightly different color or transparency. Their cross-section was rounded, or sometimes quite polygonal when there was little interstitial common opal present. The digits’ tips were often rounded. Some samples showed planar zoning of common and play-of-color opal that we interpreted as horizontal. When digit patterns were observed in such samples, they were always perpendicular to these planes (figures 10 and 14). Digit patterns were seen in nine of the 33 samples (nos. 1072, 1104, 1105, 1109, 1111, 1113, 1114, 1121, and 1122).
Some much rarer play-of-color features also have been observed in Wegel Tena opals. The cabochon in figure 15 (sample 1075) showed diffraction concentrated in points (not patches) that moved together in a synchronized fashion and changed color when the stone was tilted, or the intense pinpoint light source moved around. This revealed a perfect organization of the silica spheres that was distributed throughout the entire cabochon (pseudo single crystal; Fritsch and Rondeau, 2009). This “perfect diffraction” of light is seen only very rarely in natural gem opals. For a video and more comments on this phenomenon, see www.gemnantes.fr/research/opal/index.php#reciproque.
Another rare optical phenomenon seen in one opal that we were only able to keep for a short time was the presence of small, curved rainbows of diffraction (figure 16). Usually in play-of-color opals, each patch of diffraction is homogeneous in color. Here, the spectral colors within each patch were diffracted along a small area, ranging from 1 to 5 mm. For a video of this phenomenon, see www.gemnantes.fr/research/opal/index.php#rainbow.
SG values (before being soaked in water) ranged from 1.74 to 1.89. After immersion in water for less than one hour, some samples weighed as much as 10.2% more, resulting in higher SG values of 1.90 to 2.00 (table 2). This effect, related to the samples’ porosity, was fully reversible and repeatable.
Because of the opals’ porosity, we measured the refractive index of only five samples. In some instances, reaction with the contact liquid caused white opaque spots to form on the opal, adversely affecting the gem’s transparency and even its UV luminescence. RI values measured on sample 1073 were 1.42 and 1.44. However, we measured RIs as low as 1.36 to 1.38 on other samples (1071, 1076, and 1077). Some indices were easy to read, but most were difficult or simply impossible. Surprisingly, sample 1118a, a plate, showed an RI of 1.37 on one face and two indices on the other: 1.42 and a very sharp reading at 1.52.
UV luminescence was quite variable, ranging from bluish white to greenish white, yellow, and green. Luminescence intensity ranged from inert to strong. All brown samples or zones were inert. In long-wave UV, most non-brown samples had weak to very weak luminescence that was fairly turbid, and bluish to greenish white, with weak to very weak greenish white phosphorescence that lasted a few seconds. The typical short-wave UV reaction was slightly weaker, though the phosphorescence sometimes lasted longer. Some samples— particularly white opals with very good playof- color—luminesced more strongly, with a moderate, mostly white fluorescence. One transparent, near-colorless, play-of-color opal had a strong, pure green luminescence.
Seven transparent light color stones, with and without play-of-color, showed no reaction viewed between crossed polarizing filters. Five translucent stones with play-of-color showed cyclic variations in diffraction colors as the stones were rotated a full 360°. Two opals (no. 1076 and 1109) showed anomalous double refraction (ADR).
No spectrum was seen in the two lighter-color, transparent stones tested with the hand spectroscope. The remaining 12 milky or orange-to-brown stones showed absorption in the violet and blue regions, sometimes extending into the green. This absorption increased with the amount of light scattered by the stone (from slightly milky to white), the darkness of the bodycolor (from yellow to brown), and the thickness of the sample.
Three transparent, light-colored gems showed no reaction when viewed through the Chelsea color filter. Eleven transparent brown or strongly diffusing (milky to white) opals appeared orange to bright red.
One sample (no. 1100) contained elongated, cylindrical inclusions measuring approximately 800 μm × 1 cm (figure 17). Their surface was very irregular. EDS showed that these “tubes” were filled with silica, which may correspond to chalcedony. The outside surface of the inclusions also consisted of silica. They appeared more difficult to polish than the host opal.
Dispersed micro-inclusions of black, opaque crystals were abundant in some samples (in particular, no. 1113). EDS analyses revealed iron and sulfur, which suggests they were pyrite (pyrite usually appears black in such small dimensions).
Some of the rough opal samples were outlined by a thin layer (less than 0.1 mm thick) of black, opaque minerals. These were identified by EDS as barium-manganese oxides (probably hollandite) and native carbon (probably graphitic carbon). Also pres - ent in such layers were micrometer-sized crystals that were identified by EDS as titanium oxides (probably rutile). In rare instances, the black minerals were included in the body of the opal, filling fissures or forming dendrites.
We measured the chemical composition of several samples by EDS (both major and minor elements; see table 3) and LA-ICP-MS (trace elements; see table 4 and the G&G Data Depository). In addition to silica, we detected a significant proportion of Al (0.6–1.9 atomic %) and minor amounts of Ca (0.05–0.6 at.%), Na (up to 0.4 at.%), K (0.2–0.5 at.%), and Fe (up to 0.3 at.%). Iron was not detected in the white samples. These compositions are typical for opal (Gaillou et al., 2008a). Among the trace elements, white opal contained abundant Ba (140–226 ppm [by weight]), Sr (127–162 ppm), and Rb (44–73 ppm). The orange fire opal portion of one sample showed Ba, Sr, and Rb contents consistent with those of opal-CT (Gaillou et al., 2008a). For the concentrations of other elements, see table 4 and the G&G Data Depository.
We obtained similar spectral features for all samples (e.g., figure 18). The apparent maximum of the strongest Raman band ranged from 360 to 335 cm−1. Other, sharper Raman bands were present at ~3230–3200, 2940, 1660, 1470, 1084, 974, and 785 cm−1.
Observing the microstructure of an opal helps us characterize it and understand its growth conditions. Most often, as shown in Gaillou et al. (2008b), two main categories of structures can be observed: “smooth sphere” structure in opal-A (A for amorphous) or “lepisphere” structure in opal- CT (CT for cristobalite and tridymite; opal-A and opal-CT were originally defined on the basis of their X-ray diffraction patterns, and later on their Raman scattering patterns—see Jones and Segnit, 1971; Smallwood et al., 1997). To reveal the internal structure of an opal, one must first etch the sample in hydrofluoric acid (HF) and then observe the surface by SEM. Gaillou et al. (2008b) showed that etching in a 10% solution of HF for about 10 seconds can reveal the structure of opal. We encountered an unexpected reaction, however: Our samples were strongly affected by the acid, tending to flake away and develop networks of cracks. We modified both the concentration of HF (from 0.01% to 10%) and the duration of acid exposure (from 1 second to 3 minutes, with longer times using weaker acid), but we did not observe any organization of smooth spheres or lepispheres. Thus, we could not see any packing of spheres in opals from Wegel Tena using this technique.
We subsequently studied the structure of the opal using TEM, which revealed a regular network of spheres ~170 nm in diameter (figure 19). In another attempt to explore the structure, we studied the same sample using SEM with an unusually high voltage (15 kV) for a sample that was not coated to make it electrically conductive. Because the opal specimen was so thin, the electrons were able to pass through it to the backscattered electron detector. The similar image that was generated by this significantly different technique confirmed the TEM observations. Yet neither method helped us determine if the opal’s structure is characterized by lepispheres (as are typical of opal-CT) or smooth spheres (typical of opal-A). Regardless, the regular network of these spheres is responsible for the diffraction of visible light that results in the play-ofcolor shown by the opals.
Opals from certain localities (e.g., Querétaro, Jalisco, and Nayarit States in Mexico, and Shewa Province in Ethiopia, both opal- A and -CT) are known to destabilize, or “craze” with time. Cracks develop at the surface, and/or a white, opaque egg-like structure develops in the center of the stone (mostly in opal-CT; Aguilar- Reyes et al., 2005).
In the authors’ experience, ~5% of Wegel Tena opals develop cracks after initial sawing and preforming. Until now, out of approximately 3,000 play-of-color opal cabochons from Wegel Tena released into the market during 2008–2010 by one of us (FM), only three samples were returned after cracking. A parcel of seven opals (including sample 1072) thought to be from Mezezo was set aside in 2005 because of their unusual appearance; they are now known to be from Wegel Tena, and all the stones are still intact. Any crazing appears to be restricted to transparent material, in particular pale yellow to orange samples (fire opal) and near-colorless “crystal” opals. A few samples showed spectacular “egg” development (figure 20), as seen in some Mexican fire opal. In general, opaque white-to-yellow- to-brown opals from Wegel Tena appear very stable. There is no noticeable difference in crazing behavior between common and play-of-color opal.
There was no change in appearance (color, diaphaneity, crazing, or play-of-color) in the samples that were submitted to alternating periods of immersion in water. One customer who wears her opal constantly complained that it became more transparent when she took a shower, swam, or otherwise put her hands in water. She recognized, however, that the opal always returned to its original appearance after some time (depending on the duration of immersion)—which is due to its hydrophane character.
We noticed by accident that Wegel Tena 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.
SG was in the reported range for opal (Webster, 1975). However, some samples showed large weight gains during immersion in water, up to 10.2%. This is probably related to the high porosity of these samples, detectable simply by touching a sample with the tongue to test its “stickiness.” RI ranged from as low as 1.36 to 1.43, with one “secondary” reading of 1.52. Values as low as 1.36 have been previously measured in hydrophane from Slovakia (Reusch, 1865) and in opals from Mexico (Spencer et al., 1992). The RI sometimes varied strongly even within a single sample, depending on the orientation. Similar effects were seen in Shewa opals (Johnson et al., 1996). They are probably due to local physical or chemical heterogeneities, as commonly observed in opals.
The large patches of red and orange seen in some of the play-of-color Wegel Tena opals are not common in Brazilian and Australian opals. We found the digit patterns to be very common in Wegel Tena opals, though in some cases they were only visible with a microscope. Digit patterns were described previously only in opals from Mezezo (see figure 14 in Johnson et al., 1996; Gauthier et al., 2004). We know of only one non-Ethiopian opal with digit patterns; it was seen in Australia by one of the authors (EF). Also, Choud hary (2008) described another such opal from an unspecified source. In contrast, the planar zoning of common and play-of-color opal is often observed in opals from other deposits, volcanic or otherwise.
As expected, the spectroscope spectrum and color filter reaction were not useful for identification. The red color filter reaction, as well as the absorption of the violet and blue regions in most stones, stems from two possible factors that may combine in a given stone. Intense light scattering (the corresponding stones were milky or white) attenuates violet and blue wavelengths, as they are scattered preferentially at an angle. Also, the yellow- to-brown bodycolor is due to a continuum of absorption, increasing from the red toward the violet region. Hence it will also contribute to blocking the violet-to-blue (and even some green) wavelengths. The resulting color in rectilinearly transmitted light in both cases is orange or red, as seen with the spectroscope or the Chelsea filter. Note that this is a good example of a bright red Chelsea filter reaction that has nothing to do with the presence of chromium in the stone.
The UV luminescence properties were typical for opal. The fluorescence is a mix of intrinsic silica surface-related violetish blue emissions and extrinsic uranium-related green emission. The latter is often more visible with short-wave UV (Fritsch et al., 2001; Gaillou, 2006; Gaillou et al., 2008a).
The observed hydrophane behavior is already known for some rare opals (Webster, 1975), particularly from Opal Butte, Oregon (Smith, 1988). In the samples studied here, there was no relationship between the capacity of opal to absorb water and the change in its visual appearance (compare figure 11 and table 2).
The main Raman peaks at ~1084, 974, 785, and 357 cm−1 are typical for opal- CT and distinctly differ from those of opal-A, for which the Raman spectrum displays a main band around 420 cm−1 (Smallwood et al., 1997; Fritsch et al., 1999; Ostrooumov et al., 1999). The bands at ~3230 and 1660 cm−1 are attributed to the presence of water (Ostrooumov et al., 1999; Zarubin, 2001), and the band at 2940 cm−1 to cristobalitic water (Gaillou et al., 2004; Aguilar-Reyes et al., 2005).
The occurrence of digit patterns remains something of a mystery, but they seem to have been vertical at the time of their formation. To date, digit patterns have been observed almost exclusively in material from the two Ethiopian deposits, Wegel Tena and Mezezo. It follows, then, that the geologic conditions under which opals formed at these areas share common characteristics. However, the disposition of opal in its host rock differs between the two localities. At Wegel Tena, opal most often cements volcanic grains, while at Mezezo it usually forms nodules filling the cavities in volcanic rock. The latter is the case for most volcanic opal localities, such as Mexico and Oregon. Also, the inclusion scenes observed in opals from Wegel Tena are different from those observed in opal from any other deposit (e.g., Gübelin and Koivula, 1986).
At Wegel Tena, the opal-mineralized layer is concordant with volcano-sedimentary deposits that extend for hundreds of kilometers. No systematic prospecting for opal has been conducted, but a local miner reported to one of us (EB) that opal samples have been found in the same geologic unit and at a similar depth on the flank of the Great Rift Valley, 40 km from the present workings (Yapatsu Purpikole, pers. comm., 2009). We believe, then, that the extent of the opal-bearing layer is probably much greater than what is known today.
From our preliminary measurements of two samples, trace-element composition was comparable to opals from Mezezo (Gaillou et al., 2008a), with the following notable exceptions: Y (0.07–0.3 ppm), Nb (0.3–2.8 ppm), and Th (<0.2 ppm) were lower in the Wegel Tena opals, whereas Sc (1.5–2.3 ppm), Rb (39–73 ppm), Sr (72–162 ppm), and Ba (82–226 ppm) were higher. The enriched Ba concentration in opals from Wegel Tena is surprising. Looking at opals from elsewhere (Gaillou et al., 2008a), we see that those that formed in a volcanic environment always have low Ba (<100 ppm), whereas those from a sedimentary environment contain 100–300 ppm Ba. However, we note that the relatively high Ba contents in some Wegel Tena opals are consistent with the geologic environment that also resulted in the presence of Ba-Mn oxides in fissures mentioned above.
Areas of different composition within single samples raise the question of their petrogenesis. The orange opal is richer in Fe and presents a typical opal-CT trace-element composition (compare with Gaillou et al., 2008a), whereas the white opal has unusually high contents of Ba, Sr, and Br compared to volcanic opals.
Digit patterns are typical of Ethiopian opal, from either Mezezo or Wegel Tena, regardless of the bodycolor. The digit patterns somewhat resemble the columnar structure observed in synthetic opals (such as Gilson), but they also have significant differences. Digit patterns are made of two materials: one transparent with play-of-color and the other more turbid, often without play-of-color. They form rounded columns, whereas the columns in synthetics are far more regular and polygonal. Hence, careful observation is sufficient to distinguish the natural from synthetic opal.
To our knowledge, white opals from Wegel Tena represent the only example of opal-CT with a Ba content greater than 100 ppm. Therefore, the combination of a satisfactory Raman spectrum with trace-element analysis makes it possible to identify white opals from this locality. The fire opals displayed a chemical composition comparable to that of opal-CT from other localities.
Translucent opals from Wegel Tena resist hydration/dehydration cycles much better than opals from Mezezo, which crack and break more easily during such tests. A stabilization process has been developed to prevent crazing of Ethiopian opal (Filin and Puzynin, 2009), but in our experience this appears unnecessary for translucent opals from Wegel Tena.
Wegel Tena, Ethiopia, is a source of significant quantities of high-quality play-of-color opal (e.g., figure 21), with a mostly white bodycolor that is more market-friendly than the mostly brown material from Mezezo, ~200 km to the south. The opals are found in a specific layer of a thick volcanic sequence of alternating basalt and ignimbrite of Oligocene age. Systematic prospecting is needed to assess the extent of the opal-containing layer and the production potential, which appears quite significant.
Many of these new opals showed what we describe as “digit patterns,” a common feature in Ethiopian opals. The Wegel Tena samples typically showed the Raman spectrum of opal-CT, as do most opals that formed in a volcanic environment. Among the inclusions, we noted cylinders of silica (probably chalcedony), black microcrystals (probably pyrite), and coatings/fissures filled with bariummanganese oxides and graphite-like carbon. The combination of an enriched Ba content in the white opals (>100 ppm), a Raman spectrum typical of opal- CT, and the presence of digit patterns characterizes opals from Wegel Tena. Further trace-element analyses are necessary to strengthen the chemical criterion for Ba. The opaque-to-translucent opals from Wegel Tena are much more stable than those from Mezezo, which have a tendency to craze. Also, physically they are surprisingly tough. The Wegel Tena area has the potential to become a leading supplier of high-quality white play-of-color opal.
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