corundum, UK.12, Scotland.12, geology.11, gems

An occurrence of unusually large Scottish sapphires is described from a xenolith-bearing monchiquite dyke at Loch Roag, Isle of Lewis, in the Outer Hebrides. The dyke intrudes the Lewisian Complex and is exposed over approximately 24 m, with a width varying from 0.5 to 1.5 m. Its coarse central zone contains xenoliths of presumed mantle and deep crustal origin together with corroded megacrysts of biotite, augite, apatite, feldspar and scarce corundum. Sapphire crystals average 2–3 cm and reach 46 mm in diameter, substantially exceeding the sizes generally recorded from earlier Scottish occurrences. They commonly form distorted, striated, truncated hexagonal pyramids and bipyramids, with occasional tabular prismatic crystals. Smaller specimens tend to be fine blue, whereas larger crystals commonly contain patchy blue, green and occasional yellow areas. Fracturing is universal, often accompanied by secondary iron staining, and some crystals show basal parting. One specimen displays a hexagonal radial structure and sector zoning attributed to symmetrically distributed hematite exsolution lamellae. Detailed inclusion study was restricted because most crystals were retained as mineral specimens. A faceted 2.7 ct sapphire contained a feldspar grain, opaque breadcrumb-like inclusions and dark wispy veils associated with fractures; niobium-rich rutile is also reported in some specimens. Measured refractive indices are 1.762–1.767 for the extraordinary ray and 1.772–1.775 for the ordinary ray, with birefringence of 0.008–0.010 and specific gravity of 3.95–4.05. The sapphires show pronounced pleochroism, a broad diffuse absorption band centred near 450 nm and no fluorescence under long-wave or short-wave ultraviolet radiation. Geological comparisons suggest affinities with sapphire-bearing alkaline rocks in Southeast Asia and Australia, despite differences in age and emplacement. A proposed source is the disruption of peraluminous pegmatoids in the upper mantle or lower crust. The author considers commercial prospects limited by the small volume of exposed sapphire-bearing rock, the scarcity of crystals and the absence of demonstrated alluvial concentrations. Nevertheless, the discovery provides Scottish sapphire of cuttable size and raises the possibility that additional, perhaps wider, dykes remain concealed beneath peat.

corundum, orange sapphire, Umba, Tanzania.12, inclusions.5, gems

As known to the gemmologist and jeweller, East Africa's famous gem locality, Tanzania, is well known not only for garnets of various colour varieties, but also for corundums of many colours and hues. The occurrence of corundum in Tanzania is reported by many researchers (Zwaan, 1974; Dolenc, 1976; Amstutz & Bank, 1977; Bank, 1978). However, these reports mainly describe the common corundum varieties such as ruby and blue and yellow sapphires. Recent observations made by the author on new occurrences of sapphires from Umba Valley revealed a reddish-brown colour variety which has often been termed as 'padparadschah* in the trade. It therefore became necessary to investigate systematically the differences between the 'real' padparadschah and this new occurrence. The chemical and physical properties of the latter are given with special emphasis on the cause of colour. Further, the inclusions are reported, which may be of help in documenting the locality.

gems, Ceylon, Sri Lanka, taaffeite, taprobanite, nomenclature

A discussion of the mineralogical nomenclature of taaffeite and taprobanite. Additional abstract: Chemical and crystallographic study of material from Sri Lanka, the U.S.S.R. and China showed taaffeite and taprobanite to be the same mineral, with composition BeMg₃Al₈O₁₆ and hexagonal symmetry. Schmetzer uses the case to examine whether mineral nomenclature should give priority to type material or to an original published description.

gems, jade, jadeite, fei cui, Mesoamerica, Mexico, Guatemala, Costa Rica, Mayans, Olmecs, Aztecs, archaeology, history, archaeogemology, art, art history

The art of the peoples of Mexico and Central and South America before the Spanish Conquest in the sixteenth century has only recently attracted the attention it deserves. Regarded during the nineteenth and early twentieth centuries as “barbaric,” “crude,” and “primitive,” the art of these ancient cultures has now gained recognition as a novel, original, and sophisticated achievement. Pre-Columbian Art is a brilliant synthesis of the many artistic traditions that flourished over a vast territory and over a long span of time. The profusion of styles, techniques, and materials, and the virtual absence of historical documentation make the exploration of the subject enormously complex. The book opens with a wide-ranging investigation of the geographic, environmental, and climatic conditions that shaped the early American Indian cultures. Physical anthropology and languages are then examined, as are the social, political, and historical forces that exerted such tremendous influence on artistic expression. Next is a vivid account of the nearly five centuries of European explorations on the American continent, the earliest of which were animated by greed for riches and lust for adventure, and the most recent of which serve as models of scientific methodology. The core of the book is a detailed presentation of the development of Pre-Columbian cultures, beginning with the Stone Age hunters who painted and carved intricate symbols on the walls of caves. New light is shed on the Preclassic Olmecs, whose mysterious stone sculptures still puzzle scholars, and our understanding of the supremely beautiful and sophisticated art of the Classic Maya is enriched and deepened. As the cultures evolved, relief carving, sculpture in stone, metalwork, urban architecture—the pyramid temples—clay figures, and masks of rare stones became more and more complex; geometric abstractions and fantastically exaggerated ornamental designs proliferated. Finally, the magnificent contributions of the Aztecs and the Incas are presented in all their glory—these last splendid empires that collapsed so quickly after the Spanish invasion. In the documentary section, the art objects of these ancient peoples are grouped by period and style. Sixty of the most important sites are described in detail in a section enhanced with hundreds of photographs, maps, drawings, and charts. Also included are representations and descriptions of Indian deities, lists of rulers, a chronological table, a selected bibliography, and an index. Over 1,000 superb illustrations, including 177 plates in rich colour, clearly demonstrate the outstanding originality and inventiveness found in Pre-Columbian art. José Alcina Franch was a professor in the Department of American Anthropology and Ethnology at the University of Madrid and also lectured at the universities of Valencia and Seville. He participated in important excavations in Peru, Guatemala, and Ecuador.

gems, jade, pyroxene jade, fei cui, jadeite, dyed jade, treated jade, dyed blue jade, dyed blue jadeite

The Los Angeles laboratory was recently asked to identify the 8.98-ct translucent blue cabochon shown in figure 13. To the unaided eye, the interwoven structure typical of jadeite was apparent. A refractive index obtained by the “spot” method revealed a value of 1.66. Spectroscopic analysis was performed, revealing a sharp line at 4370 Å in the violet-blue region and the characteristic dye band centered at 6500 Å in the orange-red region of the visible spectrum. This blue cabochon was therefore identified as jadeite jade, treated color. As with other materials that are frequently dyed or stained, jadeite can be dyed virtually any color, but green is the one most commonly encountered.

corundum, USA.12, Montana.12, gems

Intergem Inc. of Aurora, Colorado, pres-ently owns the sapphire deposit in Yogo Gulch, Mon-tana. In 1983,the mine produced approximately 200,000 ct of sapphire in limited surface mining. (Thepercentage of this material that was of cuttable gem quality was not reported.)Once it has completed careful exploration, the company plans to put the mine into full operation.

Tanzania, scapolite, gems

Uses heating experiments and optical observations to refine the interpretation of coloured inclusions and post-crystallisation conditions in scapolite from central Tanzania. The study follows earlier investigations of gem-quality Tanzanian scapolites noted for their homogeneity, high birefringence and occasional chatoyancy. Parallel needle-like channels had previously been associated with reddish iron oxide or hydroxide material and considered responsible for the cat’s-eye effect. Electron-microprobe work subsequently showed that material occupying the growth tubes could have the same composition as the surrounding scapolite, while the yellow and brownish-red laminae within them represented separate mineral species. Yellow laminae were tentatively assigned to lepidocrocite from their appearance, colour and crystal habit. The reddish scales could be either maghemite or hematite, potentially produced through dehydration of lepidocrocite under different temperature conditions. To constrain this distinction, the authors draw on experimental observations of transparent scapolites from several geological settings. Refractive indices were measured during heating to 450 °C and again at room temperature after heating to temperatures as high as 800 °C, below the stated decomposition limit of scapolite. Both principal refractive indices increased approximately linearly up to about 350–375 °C, with a slightly greater increase in the extraordinary index. The magnitude of the change was approximately related to meionite content. At higher temperatures, a sharper increase in both indices was attributed to water loss. Measurements after cooling yielded nearly constant room-temperature values across the investigated heating range. Colour behaved differently: its intensity progressively diminished, and specimens became colourless at approximately 350–400 °C. The authors apply this experimentally observed colour loss to a faintly yellow Tanzanian specimen containing reddish laminae. Because its colour remained intact, they infer that it had not experienced comparable high temperatures after formation. This inferred thermal limit favours maghemite over hematite as the identity of the reddish material in its growth channels. The paper therefore combines analytical observations, experimental colour changes and proposed iron-mineral transformations to narrow an otherwise uncertain inclusion identification and reconstruct part of the specimen’s thermal history.

corundum, synthetic corundum.7, flux, Verneuil, pulling, Czochralski, hydrothermal, gems, synthetic beryl, synthetic emerald, synthetic chrysoberyl, synthetic alexandrite, YAG, GGG, synthetic rutile, synthetic cubic zirconia

Featuring color photos by E.J. Gübelin, this is a basic gemological guide to the major synthetic gem materials.

corundum, properties.3, gems

The Gemtek Gemmologist is evaluated as a thermal probe intended to distinguish not only diamond from its simulants but also gemstones with much smaller differences in thermal response. The report questions the customary description of such instruments as thermal conductivity testers, noting evidence that their responses relate to thermal inertia. The Gemmologist measures the time taken for its heated probe tip to cool between two predetermined temperatures after contact with a specimen, converting this interval into a digital count; lower readings indicate more rapid heat transfer. Reliable operation requires calibration for the working environment, a restricted ambient temperature range, specimens warmed to near body temperature, and consistent perpendicular probe contact and pressure. Tests were conducted after calibration, with specimens initially warmed against the wrist and measured ten times in succession. Most stones weighed between 0.75 and 4 ct, and supplementary tests across a wider size range indicated no size effect under the conditions examined. Results demonstrated separation of ruby from garnet, aquamarine from blue spinel and topaz, and sapphire from blue tourmaline. The natural emerald and Gilson flux-grown synthetic emerald tested also gave clearly different readings. However, many materials produced overlapping ranges, reflecting both similar thermal properties and limited repeatability. The manufacturer’s suggestion that heat-treated stones should read 10–15 points higher than untreated stones could not be confirmed with the available samples. Specimen temperature proved especially important for materials of medium or low thermal conductivity: garnet and cubic zirconia gave substantially lower readings at room temperature than at body temperature, while insufficiently warmed corundum could produce readings within the diamond range. Diamond and the metals tested were comparatively insensitive to this temperature difference. Construction and operating procedures are described, including the spring-loaded probe, indicator lamps, audible signal and user-accessible calibration adjustments. The instrument is judged capable of supplying useful supplementary identification data, particularly for diamond, its simulants and small mounted stones inaccessible to conventional optical measurements. For gemstones within its measurable range, the standard refractometer offers greater precision. The report concludes that the Gemmologist’s demanding operating conditions can produce misleading results for inexperienced users, despite its intended accessibility to people with limited gemmological knowledge.

corundum, synthetic corundum.7, flux, Verneuil, gems

A group of orange synthetic corundum crystals supplied by Tom Chatham is described, with particular attention to crystal habit, inclusions, luminescence and colour stability. The specimen weighs slightly more than 13 ct and consists of several crystals reaching approximately 1 cm in height. They display a basic rhombohedral form and are less bladed than blue synthetic sapphire crystals previously produced by the same firm. Their attractive reddish-orange colour shows some zoning, although this is less pronounced than in the blue material. The crystals are thick enough for faceting, but numerous inclusions are present, particularly angular flakes large enough to be seen without magnification. These are compared with inclusions identified as platinum in an earlier study by R.E. Kane. A substance containing numerous gas bubbles occurs on the back of the group. Drawing on Kane’s account and information from the manufacturer, O’Donoghue explains that such material is a silica-based ceramic glaze applied to crystal groups and fired at 1000 °C, a practice reported for both orange and blue Chatham groups. The most striking observations concern the response to filtered illumination and the spectrum. When illuminated by a high-intensity lamp through a copper sulphate solution and viewed through a red gelatine filter, the specimen glows strongly reddish orange. A similar but weaker response occurs under long-wave ultraviolet radiation. Examination with a hand spectroscope reveals a conspicuous sharp emission line in the deep red, readily visible even to inexperienced observers. No colour change was observed after several days in daylight or after ultraviolet exposure for one hour on each of two occasions. The author distinguishes these observations from previously reported X-ray-induced colour changes in some blue Chatham sapphires. Strong pink-orange and brownish-yellow pleochroism broadly agrees with Kane’s description. Specific gravity and refractive indices were not measured on this specimen; values quoted from Kane’s separate specimen are approximately 4.00 and 1.762–1.770, with birefringence of 0.008. The note supplements the earlier investigation through observations on another crystal group while distinguishing directly observed properties from previously published measurements.