Gem corundum, a minor but persistent megacryst in east Australian basalt fields, is mined from some placer concentrations. Laser ablation, inductively coupled plasma mass spectrometry analyses and O isotope determinations on a colour range of corundum from different fields, show that chromophore (Fe, Cr, Ti, V) and genetic indicator (Ga, Mg, δ¹⁸O) values can distinguish corundum sources (magmatic, metamorphic and metasomatic) before basalt incorporation. They also characterise corundum groups from different fields. This identified two metamorphic groups, one carrying ruby at Barrington Tops, and a magmatic group distinct from those from other gem fields (lower Fe, northeast Tasmania; higher Fe, Yarrowitch). Ruby-bearing groups show clear provincial characteristics and include lower temperature spinel-facies groups (Barrington, Yarrowitch) and higher temperature garnet-facies groups (Cudgegong–Macquarie River). High Mg/Fe and Ni values in the latter approach those for corundum in diamond, and are a possible diamond indicator. The corundum derived from diverse fold-belt and felsic sources in underlying lithosphere forms a dataset for comparing corundum from other basalt fields.

Mogok gem corundum samples from twelve localities were analyzed for trace element signatures (LA-ICP-MS method) and oxygen isotope values (δ¹⁸O, by laser fluorination). The study augmented earlier findings on Mogok gem suites that suggested the Mogok tract forms a high vanadium gem corundum area and also identified rare alluvial ruby and sapphire grains characterised by unusually high silicon, calcium and gallium, presence of noticeable boron, tin and niobium and very low iron, titanium and magnesium contents. Oxygen isotope values (δ¹⁸O) for the ruby and high Si-Ca-Ga corundum (20‰–25‰) and for sapphire (10‰–20‰) indicate typical crustal values, with values >20‰ being typical of carbonate genesis. The high Si-Ca-Ga ruby has high chromium (up to 3.2 wt % Cr) and gallium (up to 0. 08 wt % Ga) compared to most Mogok ruby (<2 wt % Cr; 6), and with other features suggest a potential skarn-like, carbonate-related genesis with a high degree of magmatic fluid input The overall trace element results widen the range of different signatures identified within Mogok gem corundum suites and indicate complex genesis. The expanded geochemical platform, related to a variety of metamorphic, metasomatic and magmatic sources, now provides a wider base for geographic typing of Mogok gem corundum suites. It allows more detailed comparisons with suites from other deposits and will assist identification of Mogok gem corundum sources used in jewelry.

Ruby in diverse geological settings leaves petrogenetic clues, in its zoning, inclusions, trace elements and oxygen isotope values. Rock-hosted and isolated crystals are compared from Myanmar, SE Asia, and New South Wales, East Australia. Myanmar ruby typifies metasomatized and metamorphic settings, while East Australian ruby xenocrysts are derived from basalts that tapped underlying fold belts. The respective suites include homogeneous ruby; bi-colored inner (violet blue) and outer (red) zoned ruby; ruby-sapphirine-spinel composites; pink to red grains and multi-zoned crystals of red-pink-white-violet (core to rim). Ruby ages were determined by using U-Pb isotopes in titanite inclusions (Thurein Taung; 32.4 Ma) and zircon inclusions (Mong Hsu; 23.9 Ma) and basalt dating in NSW, >60–40 Ma. Trace element oxide plots suggest marble sources for Thurein Taung and Mong Hsu ruby and ultramafic-mafic sources for Mong Hsu (dark cores). NSW rubies suggest metasomatic (Barrington Tops), ultramafic to mafic (Macquarie River) and metasomatic-magmatic (New England) sources. A previous study showed that Cr/Ga vs. Fe/(V + Ti) plots separate Mong Hsu ruby from other ruby fields, but did not test Mogok ruby. Thurein Taung ruby, tested here, plotted separately to Mong Hsu ruby. A Fe-Ga/Mg diagram splits ruby suites into various fields (Ga/Mg < 3), except for magmatic input into rare Mogok and Australian ruby (Ga/Mg > 6). The diverse results emphasize ruby’s potential for geographic typing.