Top Quality Ethiopian Opal Cabochon - Handmade Sterling Silver Ring Beautiful cabochon showing a continuous play of colour Classic Rocks and Gems 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. Some natural opals, mostly from Ethiopia, show a macroscopic finger-like structure called a digit pattern. This pattern consists of vertical columns that are more or less parallel, separated by a homogeneous matrix of different color, transparency, or play-of-color. This study proposes that digits develop through: (1) the deposition of a homogeneous opal layer and subsequent polygonization in the form of vertical columns; (2) preferential alteration of this layer at the vertical grain and sub-grain boundaries, creating the digit shape; (3) precipitation of a new silica gel in the space between the digits; and (4) the drying and solidification of the opal. Although polygonization in the form of vertical columns is a growth process typical of synthetic opal, the post-growth alteration of these columns into digits and the deposition of matrix are observed only in natural opal.Opal is a poorly crystallized or amorphous hydrated silica formed through the solidification of a silica gel (Jones and Segnit, 1971). The most valuable variety is “precious opal,” which displays play-of-color: patches of pure spectral colors from violet to red flashing over the stone as it is tilted. Precious opals are mined in many parts of the world, most notably in Australia, Brazil, Mexico, and Ethiopia. Other sources include the United States, Honduras, and Java. Ethiopia has been a major producer since the 2008 discovery of abundant opal at Wegel Tena, in the northeastern Wollo Province (Rondeau et al., 2009, 2010; Mazzero et al., 2009, 2010). Play-of-color arises from the diffraction of visible light on monosized, well-ordered silica spheres in opal-A (Sanders 1964; Darragh and Sanders, 1965), and lepispheres in opal-CT (Flörke et al., 1976) of appropriate diameter. Most often, play-of-color involves several juxtaposed patches of various diffraction colors. In rare cases, a network of silica spheres is distributed over the whole stone, so that the color patches move in unison. Such samples are considered natural “photonic” crystals. Whereas a crystal sensu stricto diffracts X-rays, a photonic crystal diffracts wavelengths in the visible range of the spectrum, giving rise to visible play-of-color. The diffraction colors in precious opal can be arranged in a variety of patterns, including intense specks (pinfire), flames, bands, and juxtaposed polygons (harlequin opal). Straight black lines or bands often cross the patches, and these are due to polysynthetic or mechanical twin planes of the photonic crystal, merging at the surface of the gem (Kinder, 1969; Gauthier, 1985). Unlike precious opal, common opals do not display play-of-color, usually because the silica spheres lack regular packing (Gaillou et al., 2008). Although some rare opals are colorless, most specimens present a bodycolor: white, black, gray, brown, yellow to orange (as in fire opal), red, pink, blue, green, or violet. These colors are typically due to minute mineral inclusions colored by transition metal ions that absorb part of the visible spectrum of light. These include iron for yellow to orange to brown (fire opal), copper for a saturated blue (“Peruvian opal”), and nickel for green (chrysopal; Fritsch et al., 1999). Other causes of color in the inclusions are color centers (purple fluorite inclusions) and organic compounds (quinones in pink opals; Mathey and Luckins, 1998; Fritsch et al., 2004). Figure 1. Digits in an Ethiopian opal from Mezezo, in Ethiopia’s Shewa Province. Digits are rounded, finger-like columns of opal embedded in a matrix of opal with a different appearance. This 1.5 cm stone shows rounded patches on one side (left) and columns on the other side (right). Photos by F. Mazzero. In this article, we document an optical feature encountered in some gem opals: Viewed in one direction, the surface shows a mosaic of polygonal to rounded patches of opal, separated by a homogeneous matrix of distinctly different opal. When viewed from the perpendicular direction, these patches appear more elongated and parallel, like columns, that are rounded at one end (figure 1). Each column represents a grain made up of a homogeneous network of silica spheres. The resulting three-dimensional feature’s resemblance to fingers (at the millimeter scale), inspired us to call them digits (Gauthier et al., 2004; Rondeau et al., 2010). This feature has also been described by other gemologists (Hainschwang, 2006; Choudhary, 2008). Digits are most spectacular when the rounded columns possess play-of-color and the matrix is common opal, as shown in figure 1. Digits are so frequently observed in Ethiopian opals, either from Wegel Tena in Wollo Province (Rondeau et al., 2010) or from Mezezo in Shewa Province (Johnson et al., 1996; Mazzero, 2003) that they have become the industry’s unofficial identifier for Ethiopian opal. Examples have occasionally been reported from other deposits, such as Virgin Valley, Nevada (Gübelin and Koivula, 2005; Gaber, 2007). Digits have been reported in only one Australian sample (figure 2). This paper aims to provide further documentation on digit patterns by proposing a model for their formation.
Posted on: Sat, 13 Dec 2014 20:43:53 +0000
Trending Topics
Recently Viewed Topics
© 2015