Following are excerpts from a letter written by Prof. Dr. H.A. Hänni, FGA, Director of the SSEF, concerning emeralds:
"Gemstones, and especially emeralds, may have fractures and fissures which occurred during the stones' natural history. Such open fissures are usually filled and have been since a long time artificially filled with different substances in order to enhance the clarity. In earlier days, a fracture treatment was performed with natural oils, fats or resins. Nowadays, modern filler substances such as mineral oil, artificial resins (epoxi, acryl) are usually taken since they are suitable and seem to have a better durability compared to that of historically used substances.
The identification and distinction between the different fracture filling material, even with modern analytical equipment may turn out problematic or in some cases be not possible. The most powerful tool in identification might be the Raman Laser Probe, although experience is still limited. Due to these difficulties in identification, the trade organization CIBJO has been discussing the case and is now tolerating any "colourless substance such as oil or natural or artificial resins" and accepts such treatment as common trade practice. Exempt from this new regulation in preparation are repaired (assembled) stones, stones with large cavities filled with polymerized epoxi and coloured fillers.
I would like to add my personal experience after more than fifteen years working in an important gemmological laboratory. During all these years, inspecting a large number of most valuable emeralds from various periods and different sources, I have rarely met a natural emerald larger than one carat that would not have had fissures and consequently be filled with a foreign substance."