gems, ancient gems, glyptic, archaeology, archaeogemology, ancient history, beads, species, treatment nomenclature, Near Eastern, Western Asiatic, cylinder seals

A system of nomenclature for the consistent description of archaeological artefacts made from quartz has been developed, based upon published mineralogical texts. The system has been found to be useful in the description of cylinder seals from Western Asia and the results obtained suggest that the provision of consistent identifications may be valuable in tracing the evolution of material usage over time.

gems, spinel, Tanzania, Morogoro, inclusions, spinel inclusions, högbomite inclusions

Lamellar inclusions in gem spinels reportedly originating from the Morogoro area of Tanzania are identified as an iron-free, titanium-rich member of the högbomite series. The investigation concerns rough and faceted material obtained through several independent trade sources following the first receipt of samples in June 1987. The crystals include red, pink, orange, reddish purple, purplish blue and blue octahedra reaching approximately 1 cm. Most suppliers independently reported Morogoro as the source, and the occurrence of similar inclusions across the parcels supported a common locality; an earlier attribution to the Umba Valley was therefore considered probably incorrect. Conventional physical properties, trace-element compositions and absorption spectra did not distinguish these stones significantly from natural spinels from Sri Lanka and Myanmar. Their colour-related absorption was attributed to chromium, vanadium, iron in different valence states and cobalt, with cobalt detected by X-ray fluorescence despite concentrations below the electron microprobe detection limit. In contrast, approximately 70% of the examined Tanzanian samples contained characteristic lamellar inclusions. These occur in four orientations parallel to the octahedral {111} faces of the spinel host. Viewed along a ⟨110⟩ direction, two sets intersect at angles of 109.47° and 70.53°. Immersion microscopy with crossed polarizers established that the lamellae are doubly refractive. Their extreme thinness prevented successful identification by electron microprobe, but transmission electron microscopy of ion-beam-thinned petrographic sections resolved their structure and composition. Individual lamellae measured 0.01–0.1 µm thick. Electron diffraction indicated trigonal, rhombohedral symmetry, with unit-cell dimensions of approximately a = 5.7 Å and c = 55.8 Å. Quantitative energy-dispersive X-ray analyses using a 15 nm electron probe identified magnesium–aluminium–titanium oxides, supporting their assignment to iron-free, titanium-rich högbomite. The structural relationship between högbomite and spinel is discussed in terms of titanium substituting for magnesium, with related structures also occurring in taaffeite and musgravite. The authors propose these oriented, birefringent inclusions as a practical indication of natural Tanzanian spinel, potentially useful in distinguishing such material as flux-grown synthetic spinels become increasingly available.

corundum, sources, Sri Lanka, gems

corundum, properties.3, yellow sapphire, gems

gems, spinel, corundum, sapphire imitation, Vietnam, blue spinel

corundum, sapphire, Madagascar.12, gems

corundum, China.12, treatments.6, gems

Wang Chuanfu, Yang Yaoshan, and Li Guoxun compare conventional heating with an electrolytic oxidation treatment intended to improve the appearance of dark sapphire from Shandong Province, China. Much of the available corundum is described as unsuitable for gemstones because of its dark colour and pronounced pleochroism, appearing bluish violet perpendicular to the crystallographic c-axis and green or greenish yellow parallel to it. High-temperature heating can improve apparent clarity but commonly produces commercially undesirable bluish-grey or grey stones. The authors investigate whether stronger oxidation at a substantially lower temperature can lighten the material while retaining a desirable blue hue. Their alternative treatment uses a molten fluoride electrolyte containing cryolite, aluminium fluoride, calcium fluoride, titanium dioxide, and alumina near its saturation concentration. Sapphire specimens are attached to a platinum anode and immersed in the bath at 940 °C, with a carbon crucible serving as the cathode. Electrolysis is conducted at 3.0 V for 30–45 minutes. For comparison, other specimens are heated in carbon crucibles in air at 1,700 °C for ten hours. Treated samples are prepared with parallel polished faces and their absorption spectra measured perpendicular to the c-axis. Most electrolytically treated specimens change from dark to light blue and show improved clarity without the grey component observed after conventional heating. However, slight dissolution in the molten salts causes an approximately 8% weight loss. High-temperature treatment produces only modest clarity improvement and generally bluish-grey or grey colour. The proposed explanation centres on oxidation of ferrous to ferric iron, together with differences in titanium retention. The authors associate spectral changes near 450 nm and beyond 570 nm with increased ferric iron and interpret them as evidence of oxidation. In conventionally heated specimens, diminished absorption near 550 nm and titanium detected in the crucible are taken to indicate some titanium loss, which they link to greying. At 940 °C, titanium diffusion out of the sapphire is considered more restricted; the titanium-rich electrolyte may also permit inward diffusion. Persistence or strengthening of the approximately 550 nm absorption is presented as consistent with this possibility. The study concludes that molten-salt electrolysis can improve the colour of these dark sapphires through oxidation while retaining, or potentially increasing, titanium. Its mechanistic interpretation is offered as a model based on the observed appearance, absorption spectra, and limited supporting chemical evidence.

corundum, ruby, treatments.6, gems

corundum, synthetic ruby, synthetic sapphire, Czochralski, pulling, gems

corundum, sapphire, treatments.6, surface diffusion, gems