Twenty-two gem opals from a new volcanic deposit located near Bemia, in southeastern Madagascar, were investigated by classical gemological methods, SEM-EDS, powder X-ray diffraction analysis, LA-ICP-MS, and Raman and IR spectroscopy. Although none of the opals show play-of-color, they exhibit a wide variety of hues—including those typical of fire opal—that are related to iron content, mainly from Fe-rich inclusions. Consistent with their volcanic origin, these samples are microcrystalline and composed of opal-CT or opal-C. Among the inclusions are ilmenite needles, clay minerals, and iron oxides and hydroxides. The RI and, in particular, SG values are higher than those typical of natural fire opal (e.g., from Mexico) and some synthetic fire opal, allowing for a rapid separation.
Twenty-two gem opals from a new volcanic deposit located near Bemia, in southeastern Madagascar, were investigated by classical gemological methods, SEM-EDS, powder X-ray diffraction analysis, LA-ICP-MS, and Raman and IR spectroscopy. Although none of the opals show play-of-color, they exhibit a wide variety of hues—including those typical of fire opal—that are related to iron content, mainly from Fe-rich inclusions. Consistent with their volcanic origin, these samples are microcrystalline and composed of opal-CT or opal-C. Among the inclusions are ilmenite needles, clay minerals, and iron oxides and hydroxides. The RI and, in particular, SG values are higher than those typical of natural fire opal (e.g., from Mexico) and some synthetic fire opal, allowing for a rapid separation.
