gems, corundum, Madagascar.12, basalt sapphire, Ambondromifehy

Madagascar is one of the principal producers of gem corundums recovered from continental basaltic fields. The main deposits are the secondary deposits of Ambondromifehy in the northern part of the country (sapphire-bearing palaeoplacer and placer), Soamiakatra–Mandrosohasina in the central part (primary ruby deposit, and ruby- and sapphire-bearing placer and palaeoplacer) and Vatomandry in the eastern part of the island (ruby- and sapphire-bearing palaeoplacer). 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 or associated with syenite and anorthoclasite xenoliths in the basalts. These alkali basalts transported corundum to the surface; they are linked with asthenosphere upwelling and thinning of the lithosphere underneath the Ankaratra Plateau during the Oligocene–Quaternary, and to some E–W and N–S tectonic extension structures. Oxygen isotopic compositions of the rubies from placer deposits with δ¹⁸O = 3.1 ± 1.1‰ are typical of ruby in mafic and ultramafic rocks (1.25 < δ¹⁸O < 6.8‰). The sapphires from placer and palaeoplacer deposits have δ¹⁸O values of 4.5 ± 0.5‰, which coincides with the range in sapphire-bearing syenitic rocks (4.4 < δ¹⁸O < 8.3‰). Solid inclusions such as pyrochlore, samarskite, uraninite and anorthoclasite in the sapphires confirm their syenitic origin. Zircons associated with the sapphires gave U–Pb ages younger than 35 Ma, coeval with the volcanic event. The zircon inclusions in rubies of metamorphic origin associated with the mafic and ultramafic xenolith rocks gave a Pan-African U–Pb age around 750 Ma. Nevertheless, dating of zircon crystals in situ within the anorthoclasite xenoliths has revealed two different ages, 35 and 17 Ma, which are slightly older than or contemporaneous with the onset of the volcanic events. The characteristics of the corundum, their isotopic compositions as well as their ages demonstrate the existence of two distinct sources of corundum associated with basalts in Madagascar. On one hand, the rubies associated with metagabbros and garnet-bearing pyroxenites are linked to mafic and ultramafic complexes of eclogite facies in the lower crust, retrograded to granulite facies during the Pan-African event. On the other hand, the sapphires brought up to the upper crust during the Oligo-Plio-Quaternary are interpreted to be coeval with a volcanic event involving differentiated alkaline magma as revealed by their oxygen isotopic composition with syenitic signatures.

gems, diamond, Borneo, Kalimantan, Indonesia

The origin of alluvial diamonds from the four main diamond mining districts in Kalimantan was studied through characterisation of their properties, and determination of PT and age of formation of representative collections of diamonds from four localities of the island. The diamonds are mostly colourless, yellow or pale brown, shiny surfaced, dodecahedroids, octahedron/dodecahedroids, and more rarely cube combination forms. They are intensively resorbed. They have surface radiation damage and show abrasion features indicative of fluvial transportation and crustal recycling. The diamonds were polished down to expose internal structures and mineral inclusions. The majority of the diamonds are internally homogeneous or have simple octahedral zonation and show plastic deformation. Analysis by Fourier transform infra red spectroscopy of their N content and aggregation characteristics shows that many diamonds are well-aggregated type IaB implying a long-term, mantle residence time and/or high temperatures of formation.

gems, quartz, cat’s eye, star

An unusual 12+ ct quartz cabochon is described that exhibits both strong chatoyancy and a distinct 10-rayed star, with the star set at approximately 45° to the chatoyant band. While asteriated quartz is typically sourced from Sri Lanka, this specimen is reportedly from Tamil Nadu, India. It has a bright medium yellow-green body color reminiscent of cat’s-eye chrysoberyl. The star effect results from oriented needle-like inclusions forming one set of rays at nearly 90° and another at roughly 60/120°, while the chatoyancy is caused by extremely fine, densely packed flaky inclusions aligned parallel to the c-axis. Gemological analysis confirmed that the chatoyancy and asterism are distinct optical phenomena. Although EDXRF chemical testing did not conclusively identify the inclusions, FTIR spectroscopy and other gemological tests confirmed the cabochon as natural quartz.

gems, corundum, inclusions.5, Sri Lanka.12

The turbid/translucent, near colorless (milky) metamorphic sapphire samples from Sri Lanka have been characterized after the heat treatment in N₂ at 1650 °C. As-received sapphire specimens became bluish-colored and exhibited more clarity after the heat treatment. It was found that the color change at inclusions zoning region is attributed by the dissolution. As-received samples contain the micro/nano inclusions such as rutile (TiO₂), ilmenite (FeTiO₃), spinel (MgAl₂O₄)/ulvospinel (Fe₂TiO₄) and apatite (Ca₅(PO₄)₃), which were dissolved by the heat treatment and form the blue color through Fe²⁺/Ti⁴⁺ charge transferring. The microstructures become different because as the dissolution of apatite (Ca₅(PO₄)₃(OH,F,Cl)) in alumino silicates (Al₂SiO₅) occurred, resulting in morphological change with the appearance of (Ca, Mg, Al) silicate on the surface. Both as-received and heat-treated samples showed the rhombohedral crystal structure of Al₂O₃.

gems, beryl, emerald, Afghanistan, China, Davdar, Colombia, inclusions

corundum, Mozambique.12, gems

Preliminary resultsof rubies from the new deposit in NE Mozambique are presented. The rubies derive from an amphibolite, a metamorphic rock that is also hosting the Wnza deposit in Tanzania.Their crystal habit is usually tabular to prismatic. The iron containing rubies often show twin lamellae, contain minor rutile silk, corroded amphibole grains, some zircons, fluids and secondary minerals on fissures.

gems, spinel, Tajikistan, Kuh-i-Lal, spinel inclusions

corundum, inclusions.5, gems

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 in the northern part of the country (sapphire-bearing palaeoplacer and placer), Soamiakatra–Mandrosohasina in the central part (primary ruby deposit, and ruby- and sapphire-bearing placer and palaeoplacer), and Vatomandry 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 or associated with syenite and anorthoclasite xenoliths in the basalts. These alkali basalts transported corundum to the surface; they are linked with asthenosphere upwelling and thinning of the lithosphere underneath the Ankaratra Plateau during the Oligocene–Quaternary, and to some E–W and N–S tectonic extension structures. Oxygen isotopic compositions of the rubies from placer deposits with δ¹⁸O = 3.1 ± 1.1‰ are typical of ruby in mafic and ultramafic rocks (1.25 < δ¹⁸O < 6.8‰). The sapphires from placer and palaeoplacer deposits have δ¹⁸O values of 4.5 ± 0.5‰, which coincides with the range in sapphire-bearing syenitic rocks (4.4 < δ¹⁸O < 8.3‰). Solid inclusions such as pyrochlore, samarskite, uraninite, and anorthoclasite in the sapphires confirm their syenitic origin. Zircons associated with the sapphires gave U–Pb ages younger than 35 Ma, coeval with the volcanic event. The zircon inclusions in rubies of metamorphic origin associated with the mafic and ultramafic xenolith rocks gave a Pan-African U–Pb age around 750 Ma. Nevertheless, dating of zircon crystals in situ within the anorthoclasite xenoliths has revealed two different ages, 35 and 17 Ma, which are slightly older than or contemporaneous with the onset of the volcanic events. The characteristics of the corundum, their isotopic compositions, as well as their ages demonstrate the existence of two distinct sources of corundum associated with basalts in Madagascar. On one hand, the rubies associated with metagabbros and garnet-bearing pyroxenites are linked to mafic and ultramafic complexes of eclogite facies in the lower crust, retrograded to granulite facies during the Pan-African event. On the other hand, the sapphires brought up to the upper crust during the Oligo–Plio–Quaternary are interpreted to be coeval with a volcanic event involving differentiated alkaline magma as revealed by their oxygen isotopic composition with syenitic signatures.

gems, zircon, Saxony, Germany, locality, species

Zircons from Tertiary alkali basalts in SE Saxony, Germany, were studied using electron microprobe, morphology, and inclusion analysis to determine their origin, revealing two primary alkaline-related types from the Seufzergründel placer: (I) gem-quality, honey-coloured, drop-like zircons with low trace element content, and (II) red-brown to grey, non-transparent zircons with chaotic zoning and high trace element content. A third zircon type, rich in trace elements and with clear zoning, likely derives from calc-alkaline granite. Inclusions of SO₃-rich apatite and magnesiokatophoritic amphibole support a syenitic or nepheline-syenitic source for the alkaline types. Thorite and uraninite inclusions are interpreted as exsolutions from metastable zircon–thorite solid solutions, with Th–U–Pb chemical ages yielding an “exsolution age” of 16 ± 8 to 39 ± 4 Ma. Ti-in-zircon thermometry indicates formation temperatures around 800 ± 50 °C. Zircons from related Upper Lusatian basaltic and phonolitic rocks display similar characteristics but more compositional variability. The majority of these zircons are attributed to Tertiary rift-related alkaline magmatism centered in the Eger (Ohře) Graben of North Bohemia.