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This paper provides a detailed technological characterization of the Roman opus sectile glasses (second century AD) coming from the renowned Gorga collection. Nine glass samples corresponding to imitations of the porfido verde antico, cipollino rosso, rosso antico, giallo antico, diaspro nero e giallo, semesanto and agate/alabaster stones were studied. The aim of this study was twofold: (i) archaeometric, i.e., to provide valuable data on the production, raw materials and techniques of these refined Roman glasses that mimic precious stones and (ii) methodological, i.e., to highlight the good performance of combining polished thin sections and local probe measurements for the study of glassy microstructures. Based on the nature of the flux used, the glasses from the samples were classified as either natron-type or mixed-type (natron/plant ash). The latter stem from remelted glasses and contain relict grains of wollastonite that were not found in the pure natron samples. Relict wollastonite crystals appear to be a proficient petrographic marker to spot recycled glasses along with the commonly used chemical fingerprints. Different production and colouring techniques were identified, even for a given type of imitated stone. Metallic Cu, Ca antimonates and Pb-Fe antimonates were the three opacifiers used for the opaque glasses. Based on the crystal morphologies, metallic Cu and Ca antimoniates were possibly synthesized in situ simultaneously with the glass, whilst the Pb-Fe antimonates were prepared ex situ. The working temperatures for these glasses were estimated within the 900–1100 °C range based on the presence and known thermal stability of some identified crystal phases. [ABSTRACT FROM AUTHOR]

Owing to the high market values of natural sunstones in Oregon, a kind of artificially diffused red feldspar exhibited at the Tucson Exhibition at the beginning of this century, whose color origin is the same as that of natural sunstone (copper nanoparticles). However, the details of the artificial diffusion process are less disclosed, there is no systematic method to obtain such gemstones. In this paper, we developed the high-temperature copper diffusion process for the surface recoloring of faceted labradorites, which are partly buried in the diffusant. By optimizing the experimental parameters of high-temperature copper diffusion, we successfully recolored the faceted labradorites to red and light red. The gemological and spectroscopic characteristics of the recolored faceted labradorite were further characterized. The red and light-red faceted labradorites exhibited the unique surface plasmon resonance absorption peaks of copper nanoparticles near 580 nm, which is the origin of red color. The typical inclusions formed in the faceted labradorite is in the shape of "fire cloud". The interface of red and light-red faceted labradorite that is in contact with the diffusant is less contaminated, we believe that the contamination could be further reduced or eliminated by optimizing the high-temperature copper diffusion process. The way that the sample is in contact with the diffusant partly is versatile and promising in the surface treatment of materials that have already been processed. [ABSTRACT FROM AUTHOR]

Ruby (red corundum) is one of the most prominent colored gemstones in the world. The highest-quality ruby ("pigeon blood" ruby) comes from marbles of the Mogok Stone Tract in central Myanmar. Although Mogok ruby has been exploited since the 6th century AD, the formation time of this gemstone is ambiguous and controversial. In this paper, we describe a mineralogical, geochemical, and geochronological study of ruby and titanite in ruby-bearing marbles obtained from an outcrop in the Mogok Stone Tract, central Myanmar. Petrographic observations have shown that titanite generally occurs in the marble matrix or occurs as inclusions in ruby. These two types of titanite exhibit identical chemical compositions. In situ secondary ion mass spectrometer (SIMS) U–Pb dating of the separated titanite from two representative samples of ruby-bearing marbles yielded lower intercept ages of 25.15 ± 0.24 Ma (MSWD = 0.26) and 25.06 ± 0.22 Ma (MSWD = 0.15), respectively. Because the closure temperature of the U–Pb system in titanite is close to the temperature of ruby growth, the obtained U–Pb ages (~25 Ma) are suggested to represent the timing of the studied ruby formation in Mogok. The acquired ages are in agreement with the timing of post-collisional extension in the Himalaya related to the migration of the eastern Himalayan syntaxis. Combining our dating results with previous geochronological data from the Mogok Stone Tract, we suggest that the formation of the studied ruby is most likely related to the high-temperature metamorphic event in the marbles during the India–Asia collision. Our study not only confirms that texturally constrained titanite could be a precise geochronometer to date the mineralization of different types of ruby, but also provides important geochronological information linking gemstone formation to the India–Asia collision. [ABSTRACT FROM AUTHOR]