corundum, synthetic corundum.7, color.4, spectroscopy, Madagascar.12, ruby, gems

Gallium concentrations in emerald, corundum and chrysoberyl are evaluated as a proposed means of distinguishing natural from synthetic gemstones. Because gallium and aluminium have similar ionic radii and the same valence, gallium can substitute for aluminium in these minerals, but its abundance depends on geological conditions and the materials used in synthesis. The study tests earlier suggestions that gallium concentrations above approximately 50–200 ppm establish natural origin in corundum, that lower concentrations indicate synthesis, and that measurable gallium identifies natural alexandrite. Neutron activation analysis results obtained by the authors and incorporated from an earlier investigation cover 32 emeralds, comprising 22 natural and 10 synthetic specimens; 18 corundums, comprising eight natural and 10 synthetic specimens; and 16 chrysoberyls, comprising six natural alexandrites, two synthetic alexandrites and eight other natural chrysoberyls. The results demonstrate overlap and important exceptions to the proposed criteria. Natural and synthetic emeralds have similar gallium ranges, with several synthetic samples containing concentrations comparable to natural stones. Four natural corundums contain less than 50 ppm gallium, while only one of the eight natural samples exceeds 200 ppm. A synthetic alexandrite contains 480 ppm, exceeding the concentrations measured in the natural alexandrites examined, whereas one natural chrysoberyl contains only 2 ppm. The paper also assesses analytical limitations that complicate interpretation. Several electron-beam and X-ray methods lack sufficient sensitivity for reliable determination of low gallium concentrations, and surface measurements may fail to represent variations associated with internal zoning or structure. Neutron activation analysis is regarded as the most suitable available technique, with a detection limit of approximately 0.5 ppm and reported accuracy of 3–10% of the measured concentration. Its routine use is nevertheless restricted by cost, lengthy analytical procedures and the slow decay of induced radioactivity. The authors conclude that gallium content alone cannot reliably establish natural or synthetic origin in the gemstone groups investigated. Proposed concentration thresholds require caution, and any identification suggested by gallium analysis must be corroborated by additional tests.