corundum, inclusions.5, gems

gems, feldspar, moonstone, albite, peristerite

This note describes transparent feldspar from Tanzania displaying blue adularescence, marketed since mid-2004 as Tanzanian moonstone. The material is identified as albite (peristerite), and its physical and chemical properties are reported. The blue sheen is attributed to Tyndall scattering. Microscopic examination reveals white, parallel, needle-like inclusions.

gems, inclusions, quartz

Photomicrographs of inclusions in quartz. The every day mineral quartz can be found all over the world. It takes a long time to form. Over millions of years, a growing quartz crystal sometimes incorporates other minerals that develop simultaneously, thus forming so called inclusions. This book presents photos of fascinating inclusions in quartz, often beautiful like a dream. It wants to give you an idea of the fantastic inner life of quartz crystals and might also be an animation for your own engagement with this phenomenon. The every day mineral quartz so gets a whole new perspective.

corundum, synthetic corundum.7, gems

Large crystals of synthetic ruby (not of gem quality) were grown by the temperature gradient technique from seed plates using Al₂O₃ powders mixed with 0.5–3% Cr₂O₃. Crystallization started at 2,050 °C; cooling rates were 0.5–2.5 °C/hour. The largest crystal measured 75 mm in diameter, 45 mm long, and weighed 1,076 g. This crystal was dark red in the center and even darker red (almost black) at its edges and bottom. Wave-like growth bands were observed in the top portion, while clouds of minute inclusions were concentrated near the seed plate. The concentration of the inclusions decreased from edge to center and from bottom to top, resulting in a “trumpet”-shaped distribution along the growth direction similar to that found in synthetic Ti-doped sapphire crystals grown by the same technique. Electron-microprobe analysis showed that the inclusions were mainly uncrystallized Al₂O₃ powder. Gas bubbles or voids also were observed. Microprobe and spectroscopic (visible range) data showed that the Cr³⁺ concentration increased in two directions (i.e., from center to edge and from bottom to top), corresponding to the growth progression. The authors propose that the quality of synthetic ruby crystals produced by this method could be improved by optimizing the cooling rate, purifying the raw materials, and controlling the temperature during growth.

gems, inclusions, quartz

corundum, geology.11, gems

Based on mineralogic and geologic research on corundum occurrences in Russia, and published data on corundum localities occurring elsewhere, it is emphasized that regional metamorphism of different rock types (e.g., marble, gneiss) is the major factor controlling the formation of corundum deposits and determining the quality of the gems they contain. The chemical and physical properties of the metamorphic host rocks ultimately determine the characteristics of gem corundum. The sapphires from Kashmir and rubies from Myanmar, which formed in metamorphic environments, are cited as examples. The superb gems from these deposits had the appropriate chromophoric elements available during their formation, under the proper metamorphic conditions. Characteristic mineral inclusions are considered indicative of specific genetic types of corundum, and, for alluvial stones, the inclusions can be used to infer the origin of the gem corundum. Gem corundums originating from other geologic sources (e.g., igneous or metasomatic) are likely to be of lower quality than those of metamorphic origin.

gems, beryl, emerald, Brazil, inclusions

Investigates solid and fluid inclusions in emeralds from Campos Verdes, Goiás, central Brazil, using scanning electron microscopy and transmitted-light microscopy. A significant finding is the identification of sylvite (potassium chloride) inclusions, providing evidence for potassium-bearing fluids during emerald formation. These inclusions also contribute to distinguishing Campos Verdes emeralds from material from some other Brazilian deposits.

corundum, inclusions.5, Thailand.12, Chantaburi, star sapphire, gems

After a brief overview of some basic facts concerning asterism, the characteristics of black star sapphires from Thailand are described. As in star corundums from other sources, these stars were found to be diffraction phenomena created by needles or lamellae of rutile or hematite-ilmenite. These inclusions can be oriented either parallel or perpendicular to the second-order prism faces.. Six-rayed star sapphires from Bang-kha-cha [also spelled Bang Kacha] show bluish white or, less commonly, “golden” yellow stars. The bluish white stars are caused by Ti-bearing hematite (as are similar black stars from Australia), whereas the nature of the mineral(s) forming the yellow stars is unknown. In the bluish white stars, the band of light (chatoyancy) can be oriented both parallel and perpendicular to the second-order prisms, but they are always parallel to these prisms in the yellow stars.. Rarely, both colors of stars are combined in the same specimen and form 12-rayed star sapphires. The authors studied nine such sapphires and found that the bluish white stars were always oriented perpendicular to the prism faces, whereas the yellow stars were parallel to these faces. The article is illustrated with four excellent photographs and two diagrams, which make the rather complicated phenomena easily understandable.

gems, garnet, andradite, demantoid, Eritrea

Characterizes demantoid garnets recovered from sandy gravels near Asmara, Eritrea. Examination of eight faceted stones and a parcel of rough totaling nearly 240 ct identified some specimens as peridot; the remainder were demantoid, with specific gravities of 3.80–3.88 and refractive indices exceeding a standard refractometer’s range. Absorption spectra show features associated with ferric iron and chromium. Horsetail inclusions resembling those in Russian demantoid were observed. The association with peridot suggests an origin in serpentinized peridotite.

corundum, sources.12, Thailand.12, Bo Ploi, inclusions.5, gems

Three main types of fluid inclusions occur in Bo Ploi sapphires: (1) vapor-rich CO₂ with a density up to about 0.86 g/cm³; (2) multiphase inclusions containing several daughter minerals, hypersaline brine, and a CO₂-rich vapor phase; and (3) (silicate?)-melt inclusions with immobile vapor bubbles in an isotropic or weakly anisotropic phase of low relief, with bubbles becoming mobile when heated above 800 °C. These observations suggest a magmatic origin for the sapphires, although they did not crystallize as phenocrysts within the basalts that transported them to the surface.