Investigation on the gemological, physical and compositional properties of some green opals from Eastern Rhodopes, Bulgaria

Abstract: The physical and compositional properties of some common opals from Eastern Rhodopes, Bulgaria have been investigated through several methodologies such as optical and gemological equipments, X-Ray Powder Diffraction (XRPD), Scanning Electron Microscopy (SEM), Raman Spectroscopy and Laser Ablation Microprobe (LA-ICP-MS). The investigated opals show different colors such as light-apple and yellowish green; the diaphaneity is opaque or opaque/translucent with luster waxy/dull/greasy. They are inert to the long and short wavelength UV radiation (366–254 nm). Refractive index and specific gravity values are between 1.445–1.470 and 1.96–2.09 g/cm3, respectively. XRPD analyses show the opals are CT type and in some cases contain quartz and clay minerals (saponite). Fe and, subordinately, V appear the main chromophores that determine the green-yellowish color, while Ni is responsible for the apple and light green. Other detected trace elements are Al, Ca, K, Mg, Na, B, Zn, Rb. SEM analysis shows three types of structures already observed in the CT opals like randomly distributed nanograins and lepispheres, mammillary structure, channels structure made up by bundles of fibers. Raman spectra confirm the samples are made up by para-crystalline CT-opal, generally considered of volcanic origin, with significative content of saponite. Graphic abstract: [Figure not available: see fulltext.]