gems, opal, Hungary, Slovakia, Australia, locality, species

Slovakian opals, hosted in andesitic rock and likely formed through water circulation during tectonic activity, exhibit physical characteristics typically associated with sedimentary opals, despite their volcanic setting. These include opal-A structure (X-ray diffraction), a main Raman peak at 437 cm⁻¹, and large silica spheres (125–270 nm), which contrast with the finer microstructures of other volcanic opals. These findings suggest that formation temperature, rather than host-rock type, primarily governs opal properties. Preliminary oxygen isotope data support this: Slovakian and Australian opals show high δ¹⁸O values (~31‰), indicating low formation temperatures (<45 °C), while Mexican opals-CT display lower δ¹⁸O (~13‰), consistent with higher-temperature formation, potentially reaching 190 °C.