Treated opal from Ethiopia

The occurrence of opal from Ethiopia has been reported in 1994 (Koivula J.I., et al 1994). Large opal deposits found in Wollo province at the first half of 2008 (Rondeu., et al 2010), large amount of cut stones are supplied to the gemstone market. But, it was also reported that export as rough gemstone has been prohibited in early 2013 (Addis Fortune Jan. 2013). Body color show range from colorless-white to pale yellow-brown and transparency show translucent to opaque. Weak blue UV fluorescence observed from almost Ethiopian opals, and it is different from Australian opals that show strong blue UV fluorescence and phosphorescence. Individual weight of Ethiopian opals can also change easily, because of the porous inner structure.<br> Black colored opals are supplied to the gemstone market from 2013, which is come from Ethiopia and similar to black opal. The seller indicates that these opals are treated by using the organic matter and acid. The prices of black colored opal are higher than not treated Ethiopian opal, and is estimated to simulate Australian black opal. <br> These stones show slightly brownish black body color with vivid play of color, it is different from the Australian black opal and the treated opal using smoke process and traditional sugar acid process. In addition, it seems that characterized by thickness of loose stones are quietly thick and absent of small loose stones. Transparent ray of black colored opal indicate dark red in color, and there are some black concentration as speckle or scratch mark. Cross section of black colored opal show color zonal structure, black matter are reached to core of stone, and surface show most dark color. Which indicate that the effect of the treatment is extend to the inside, and the treatment process need pre-form and re-polishing the surface after treatment. Origin of black color is reported as the presence of amorphous carbon (Williams 2012), we also confirmed the presence of amorphous carbon. <br> Dyed various color opal from Ethiopia began to supply since the beginning of this year.<br> We confirmed yellow-orange dyed opal which is similar to Fire opal, and Pink and blue dyed opal which are not exist in nature. Impregnation of colored resin was suspected, but results of near infrared spectroscopy analysis indicate that resin is not detected and use only dye to change color. <br> In this study, we reported characterization of treated black color opal and dyed opal from Ethiopia for gem identification, and progress of reproduction experiment and preliminary result.