Alberto Leone

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

The physical and compositional properties of some “Hungarian” opals have been investigated through several methodologies such as optical analysis, specific gravity, refractive indices, AvaSpec spectroscopy, XRPD, IR and Raman spectroscopy, LA-ICP-MS. The “Hungarian” opal deposits, now in Slovakia, represented the largest and most significant gem opal deposits in Europe from the Roman Times to XIXth century. The analysed opals include brown semiopals, white opals and hyalite. In general, chromophore elements like V, Cr, Cu, Ti, Co are present in very low concentrations and do not influence the physical properties of the opals. Elements like Na, Mg, Al, K, Ca, Sc, Cr and Fe have been detected, in variable amounts, in all the investigated samples. The semiopals can be used as ornamental and semiprecious material, are mostly made up by CT opal close to pure tridymite and lower amounts of goethite, show high trace elements contents while the REE pattern show a clear Eu negative anomaly. These opals probably formed during an hydrothermal, or late magmatic stage. The white opals are made up by amorphous opal (A), show appreciable play of color and iridescence and are surely the most relevant for gemological purposes. The most pure white opals show low contents of trace elements and a rather homogeneous composition, that can be used as geographical marker. Conversely some opals affected by contamination from host rocks show much higher trace element contents. White opals probably formed during a low temperature tectonic event. The samples of hyalite are yellowish, opaque and greasy. They are made up by CT opal show relevant content of B and As while the other trace element are quite low, and are probably of volcanic origin.

Gemological, physical and chemical properties of prase opals from Hanety Hill (Tanzania)

Chemical, physical and gemological properties of some green “prase” opals from Hanety Hill in Central Tanzania were investigated. The color of the opals ranges from clear green to apple green, diaphaneity from translucent to semitransparent, luster is vitreous, and all result inert to UV lamp radiations. Specific gravity values are between 2.11-2.13, refraction indices between 1.439 and 1.458, comparable with literature data. XRD and FTIR analyses show the opals belong to the CT type, with tridymite higher than cristobalite. SEM observations revealed a lepispheric and mammillary structure formed by spherules with a diameter around 10-12 μm, on their turn composed of amorphous silica microspheres. The opals show a homogeneous chemical composition with very high contents of Ni and lower of Mg, Ca, Fe and transition elements like Zn, Cr and Co. Al and K are nearly absent. The chemical composition of the opals reflects that of serpentine rocks. Because of its abundance, the cromophore element responsible for the green color is Ni. Prase opals may have formed through the process of low temperature metamorphism, which generated the serpentinite from a magmatic ultrafemic rock.

AVASPEC 2048: an innovative spectroscopic methodology to differentiate the natural emeralds from the synthetic ones

New and sophisticated synthetic gems are constantly introduced in the market making the identification of them very difficult, if one uses only the microscope. In this paper we present the results obtained from a new methodology, the AvaSpec-2048 spectrometer, that can help to identify the natural emeralds from the synthetic ones. The AvaSpec-2048 spectrometer is an original instrument that can acquire electromagnetic spectra in a range between 400 nm and 1000 nm (VIS-NIR), and can be used to identify gemological materials allowing an accurate discrimination of natural products from the artificial ones. The physical and optical properties, microscopic features and absorption spectra of natural and synthetic emeralds have been also investigated through others traditional methodologies such as optical analysis, specific gravities, refractive indices and Vis-NIR spectroscopy. The comparison of the absorption spectra obtained with the AvaSpec-248 spectrometer allows identifying and distinguishing the origin of the analyzed gems.