corundum, Tanzania.12, gems

Describes rubies, blue sapphires and other colored corundums from Tanzania, with particular attention to their gemological characteristics. The authors characterize the colors quantitatively using a chromaticity-coordinate diagram and examine the stones’ absorption spectra and mineral inclusions. The study compares the different color varieties and relates their optical behavior to their composition, providing analytical data useful for characterizing Tanzanian gem corundum. A contemporary Journal of Gemmology abstract specifically notes the treatment of color, inclusions and absorption spectra.

corundum, Australia.12, geology.11, inclusions.5, gems

Uranium–lead isotope dating of two zircon inclusions in sapphires from the Central Province, New South Wales, yields ages of 35.9 ± 1.9 and 33.7 ± 2.1 million years, consistent with the broader timing of volcanism in the region, as indicated by potassium–argon ages of 19–38 Ma and zircon fission track ages of 2–49 Ma; this temporal correlation, along with the frequent association of corundum with alkali basaltic provinces, supports a genetic relationship between large corundum crystal growth and alkali basaltic magma generation, although experimental efforts to crystallize corundum from corundum-bearing basaltic compositions have failed, and the inclusion of incompatible-element-rich minerals such as zircon, columbite, thorite, uranium pyrochlore, and alkali feldspar suggests that corundum crystallization requires unusually enriched melts with high volatile and incompatible element contents—conditions not typical of standard basaltic compositions—implying that sapphire formation occurs through a complex process involving residual melts or fluids enriched in volatiles and incompatible elements, with the crystallized assemblages subsequently transported to the surface by ascending magmas; the effectiveness of this process likely governs whether a basaltic province yields economically viable concentrations of sapphires.

gems, inclusions, beryl, emerald, Brazil, Santa Terezinha de Goiás

Describes the emerald deposits of Santa Terezinha de Goiás, Brazil, presenting evidence for their stratiform occurrence and documenting the stones’ gemological properties, chemistry and inclusions. Refractive indices reach 1.593–1.602, with birefringence of 0.008–0.010, reported as the highest values then observed in emerald. Chromium and iron show the highest mean concentrations among Brazilian emeralds. Fluid inclusions are generally very small. Mineral inclusions include spinel, carbonates, mica, talc, amphibole and pyrite, with rutile, hematite, chalcopyrite, pentlandite and ilmenite also recorded.

corundum, ruby, inclusions.5, Thailand.12, gems

gems, spinel, Tanzania, spinel inclusions

A brief gemological report on spinels from Tanzania’s Umba Valley, occurring in pink, red, orangish red and bluish violet colors. The specimens examined had refractive indices of 1.713–1.718 and specific gravities of 3.54–3.69. Spectroscopic examination was used to investigate the causes of the different colors. The note documents the range of gem spinel then reaching the market from the Umba Valley and provides basic properties useful for characterizing this East African material.

corundum, sapphire, Kenya.12, Turkana, asterism, inclusions.5, gems

corundum, orange sapphire, Malawi.12, gems

Describes orange gem-quality corundum from the Chimwadzulu Hills of southern Malawi, near the Mozambique border. The material has refractive indices of nε = 1.763–1.765 and nω = 1.771–1.773, birefringence of 0.008, and specific gravity of 3.97–3.98. Microscopic features include twin lamellae, healed fractures and rounded doubly refractive mineral inclusions. Spectroscopic examination indicates that the orange coloration results from the superposition of absorptions due to Fe³⁺ and Cr³⁺. The authors contrast this mechanism with orange Sri Lankan corundum whose coloration may be produced by color centers.

corundum, Inclusions.5, Burma.12, gems

inclusions, photomicrography, gems

corundum, inclusions.5, gems