Investigation on the gemological, physical and compositional properties of some opals from Slovakia ("Hungarian" opals)

The "Hungarian" opal deposits, now in Slovakia, represented the largest and most significant gem opal deposit in Europe from the Roman Times to XIX century. The physical and compositional properties of some "Hungarian" opals have been investigated through several methodologies such as optical analysis, specific gravity, refractive indices, XRPD, IR and LA-ICP-MS. The investigated opals are white, show appreciable play of color with indigo or orange-blue flashes and are surely relevant for gemological purposes. XRD and IR analyses showed the samples are made up by amorphous opal (A). The host rocks are mostly made up by an assemblage of feldspars (oligoclase and sanidine) with lower cristobalite, augite, quartz and kaolinite. The trace element contents are around 200-300 ppm; the most abundant element are Ca, Al, Na, K with lower amounts of Fe, Mg, Sc, Cr. Cromophore elements like V, Cr, Cu, Co, Ni are low or absent. The homogeneous and low trace element composition and the amorphous structure can represent an identification marker. The low trace element contents, especially for elements like Ca, Al, Na, Ba, and Sr would indicate these opals formed through precipitation from residual silica-rich hydrothermal fluids in closed systems, in condition of low temperature.