corundum, Geology.11, Madagascar.12, gems

Madagascar is one of the principal producers of gem corundums recovered from continental basaltic fields. The main deposits are the secondary deposits of Ambondromifehy from the Antsiranana Province in the northern part of the country (sapphire-bearing palaeoplacer and placer), Soamiakatra–Mandrosohasina from the Antananarivo Province in the central part (primary ruby deposit, and ruby and/or sapphire-bearing placer and palaeoplacer), and Vatomandry from the Toamasina Province in the eastern part of the island (ruby- and sapphire-bearing paleoplacer). Soamiakatra is the only known primary deposit where ruby is found in metagabbro and pyroxenite xenoliths, which were entrained and brought up to the upper crust by the Ankaratra volcanic event. Petrographic studies have demonstrated the existence of two different conditions of ruby formation at the boundary of the eclogite domain (HT ~1100 °C, HP ~20 kbar) and granulite facies (HT ~1100 °C, BP <15 kbar). In contrast, the sapphires originated from alkaline mafic magmatic chambers at the lower continental crust–mantle boundary. They occur within syenite and anorthoclasite xenoliths in the basalts or are associated with them. These alkali basalts transported corundum and their host rocks to the surface; they are linked with asthenosphere upwelling and thinning of the lithosphere underneath the Ankaratra Plateau during the Oligocene–Quaternary. The mineralogical study of ruby and sapphire permitted us to establish the relationships that exist between their chemical composition, their colour, and the nature of their solid inclusions. Iron is the main chromophore of sapphire, and the Fe²⁺/Fe³⁺ ratio in its structure, in addition to the quantity of titanium and chromium available in the fluid, determines the colour of the sapphire and the hue of the polychrome sapphires. Chromium is the second most important chromophore for pink, violet-blue, and some blue-green to light-blue sapphires. For ruby, chromium is the chromophore, but iron and vanadium interfere to produce red-brown to purplish-blue crystals. The deeply coloured, high-quality rubies are characterized by a Fe₂O₃/Cr₂O₃ ratio less than 1. The chemical diagrams used for the genetic classification of corundum indicate a metamorphic origin for all rubies from the central and eastern regions of Madagascar: rubies are hosted either by mafic and/or ultramafic rocks or by metasomatites formed by fluid–mafic rock alteration (“plumasite” or biotitite). Two processes have been identified to explain the origin of sapphires from the different regions: (i) a dominant magmatic process linked to syenites (anorthoclasite), and (ii) a metamorphic process (probably of skarn type). The oxygen isotopic compositions of the corundum corroborate these two origins. Solid inclusions such as pyrochlore, samarskite, uraninite, and anorthoclase in the sapphires confirm their syenitic origin. The presence of pentlandite, talc, sillimanite, titanite, and phlogopite in the rubies testifies to a metamorphic paragenesis.

corundum, Mozambique.12, gems

Rubies have recently been discovered in northern Mozambique, specifically in Niassa Province (M’sawize village, Lichinga district) and Cabo Delgado Province (Montepuez area). These rubies display internal features such as twin planes, fluid-film inclusions, negative crystals, and crystalline inclusions. Many of the crystalline inclusions are colorless to green amphibole (possibly pargasite), along with confirmed apatite and fine needles producing “silk.” Some fractures contain material resembling hematite. Infrared spectra of unheated Mozambique rubies show a broad absorption band at 3309 and 3081 cm⁻¹, which disappears after heating; weak bands at 2074 and 1980 cm⁻¹, likely from diaspore, may also be present. EDXRF analysis reveals Cr contents of 0.3–0.8 wt.% and Fe contents of 0.2–0.5 wt.%, higher than typical non-basalt-related rubies but lower than basaltic rubies. Most Mozambique rubies have been heat-treated, often leaving flux residues in fissures.

corundum, Cambodia.12, Vietnam.12, gems

While trapiche emerald is well known, trapiche corundum (especially blue sapphire) is considerably rarer. The authors examined 33 blue sapphires from Vietnam and 27 from Cambodia. The majority showed trapiche structure, but non-trapiche specimens were included for comparison. The trapiche sapphires typically featured a brown core and brown to yellowish brown arms separating the sections of blue crystal; dotted and needle-like inclusions were also seen. Iron and titanium were both present in the samples, but the iron was concentrated in the trapiche core while the titanium was evenly distributed for the most part. The authors also compared the visual characteristics of trapiche rubies and sapphires, as well as trapiche vs. star phenomena. The trapiche structure could point to a multi-stage growth process that starts at the core, with the arms and blue crystal segments growing concurrently but at different rates.

gems, corundum, India.12, Kashmir, sapphire

gems, corundum, quality.10, crown jewels, auction, diamond, pearl, jade, beryl, emerald

This pocket-sized work is a photo- graphic catalogue of the Crown Jewels of the Kingdom of Bohemia, which date from the reign of King Charles IV (1316–1378). The book is the product of gemological examinations conduct- ed in 1998 and 2003; a review of these examinations is also provided. Included is a CD-ROM with 600 additional photos.

corundum, Australia.12, sapphire, gems

Waterworn sapphire crystals (≤7 mm) showing a wide range of colors (green, dull blue, pale yellow, pink, purple, and pale brown) are found in concentrates from an alluvial gold mine in ferruginous gravel at Carapooee, near St. Arnaud in southeastern Australia. Chemical analysis by LA-ICP-MS suggests that these sapphires were derived from mainly metamorphic and metasomatic source rocks, with a smaller magmatic component, similar to the gem corundums from basalt-related Cenozoic goldfields in eastern Australia.

gems, spinel, Afghanistan, Tajikistan, balas ruby, Kuh-i-Lal, crystal form, crystal habit

corundum, Thailand.12, gems

corundum, Geology.11, gems

corundum, treatments.6, Thailand.12, gems