gems, photography, minerals

J.A. Scovil provides a practical guide to photographing mineral specimens, fossils, gemstones, jewellery and other lapidary objects, integrating technical instruction with aesthetic considerations and the requirements of accurate documentation. The book begins by distinguishing photography intended primarily for identification or cataloguing from more interpretive presentation, while showing how careful composition can serve both purposes. Specimen selection, orientation, framing, cleaning, support and scale establish the foundation for successful images. Equipment chapters address cameras, lenses, vibration control, medium- and large-format systems, film selection, exposure measurement and filtration within a predominantly film-based workflow. Lighting receives sustained attention as the principal means of revealing crystal form, surface texture, colour, transparency and internal features. Direct, transmitted, darkfield and coaxial illumination are explained alongside scanning-light techniques and light painting, with guidance on combining main and fill lights, reflectors, highlights and background illumination. Background materials and their placement are considered in relation to reflections, shadows, contrast and visual separation. Close-up methods cover macro lenses, supplementary lenses, extension tubes, bellows and reversed lenses, emphasizing the relationships among magnification, working distance, perspective, depth of field and specimen orientation. Separate chapters extend these principles to photomicrography, stereophotography and ultraviolet-excited fluorescence photography. Fluorescence work requires particular attention to excitation and barrier filters, unwanted reflected light, exposure bracketing and film colour shifts during long exposures. Applications to lapidary materials address the reflective surfaces of carvings, cabochons, faceted gems and jewellery, together with specialized methods for recording inclusions, optical effects and growth features. The fossil chapter places greater emphasis on unambiguous scientific illustration, using lighting direction to reveal relief and texture and discussing ways to improve contrast between specimens and matrix. Practical advice includes recording dimensions and carat weights, protecting delicate or valuable subjects, organizing equipment for work away from the studio and preparing clear slide presentations. Photographs, diagrams and examples demonstrate how changes in orientation, illumination and setup alter the information communicated by an image. Appendices describe useful homemade devices and supply sources, supplemented by a glossary, bibliography and index. Throughout, Scovil advocates deliberate experimentation and control of individual variables, treating the photographer as an interpreter whose choices should convey the specimen’s distinctive character without compromising the intended accuracy of its representation.

corundum, Madagascar.12, Andranondambo, gems

Milisenda and Henn describe the geological setting, gemological properties, inclusions and chemical composition of blue sapphires from the newly commercialized Andranondambo occurrence in southeastern Madagascar. The deposit lies approximately 100 km north of Fort Dauphin, within altered marbles and calc-silicate gneisses of the Tranomaro formation. These rocks belong to a high-grade metamorphic terrain extensively reworked during the Pan-African event. Regional geochronological evidence leads the authors to infer that corundum formation accompanied granulite-facies metamorphism approximately 600 million years ago. Sapphires are recovered with simple hand tools from small pits in residual gravels, and the occurrence is considered promising for commercial development. Studied samples are slightly to moderately included and commonly display conspicuous colour zoning, with intense blue concentrated in broad central bands. Alternating darker and paler layers and growth structures follow basal and bipyramidal planes. Heat treatment can substantially reduce the visible zoning and improve appearance. Crystal fragments preserve rhombohedral and pyramidal forms consistent with metamorphic sources, contrasting with the etched or partly dissolved surfaces often associated with basalt-transported corundum. Refractive indices are 1.761–1.763 and 1.769–1.771, maximum birefringence is approximately 0.008, and specific gravity ranges from 3.91 to 3.98. Distinct to strong dichroism combines green-blue and violet-blue hues. All samples are inert under long-wave ultraviolet radiation; most are also inert under short-wave radiation, although weak greenish-blue or uneven orange fluorescence occurs occasionally. Polarized absorption spectra show ferric-iron features and iron–titanium intervalence charge transfer responsible for blue colour. The approximately 850 nm absorption commonly observed in sapphires from basaltic sources is absent. Preliminary microprobe work identifies apatite, calcite, dolomite and an incompletely characterized clay mineral among the inclusions. Other features include probable mica platelets, apatite associated with parallel hollow tubes, negative crystals, healed fractures containing fluid inclusions and occasional lamellar twinning. Electron microprobe analyses of three sapphires identify iron as the principal trace constituent, followed by titanium, with smaller measurable amounts of silicon, gallium and chromium. The combined evidence closely resembles that of metamorphic sapphires from Sri Lanka and Myanmar, supporting the classification of Andranondambo as a significant new source of metamorphic blue sapphire.

corundum, geology.11, Australia.12, gems

Ruby–sapphire–sapphirine–spinel forms small, corroded, crystalline aggregates in corundum-bearing alluvials shed from the Tertiary Barrington basalt shield volcano. Sapphirine is near a 7:9:3 (MgO–Al₂O₃–SiO₂) composition and, together with the corundum, shows reaction rims of pleonaste spinel. Spinel in the aggregates has a compositional range Sp 68–73 Hc 27–29 Cm 0–3. The aggregates give new insights into the ruby–sapphire source rocks. Potential origins include metamorphic recrystallization of aluminous material (below 1460 °C) or high temperature–high pressure crystallization reactions related to lamprophyric or basaltic magmas (up to 1300 °C and 20 kbar). Sapphirine–spinel thermometry suggests final crystallization temperatures for the aggregates around 780 to 940 °C and reaction with host magmas at over 1000 °C. The Barrington gemfield includes two distinct corundum suites. One, typical in eastern Australia, is dominated by blue–green, well-crystallized, growth-zoned sapphire, commonly containing rutile silk and Fe-rich spinel inclusions (Hc 51–73, Mt 18–35, Mf 6–8, Usp 2–6). The other, an unusual suite, is dominated by ruby and pastel-coloured sapphires, with little crystal shape or growth zonation and restricted mineral inclusions, mostly chromian pleonaste and pleonaste. The ruby–sapphire–sapphirine–spinel aggregates provoke new thoughts on the origin of rubies and sapphires and their indicator minerals in eastern Australian and southeastern Asian volcanic gemfields.

gems, corundum, chemistry.2, advanced testing, trace element analysis, nuclear, microscopy, ruby inclusions

The concentrations of trace elements (e.g. Ti, V, Cr, Fe, and Cu) in ornamental rubies have been linked to the country of origin for natural stones [1,2]. These broad-beam PIXE results show relatively large variations between stones from one country, which might indicate that they stem from different geological environments in a country, or that there are inclusions or surface contaminations present in individual stones. PIXE elemental maps were used to find contamination- and inclusion-free regions on rubies from Mong Hsu, Burma, and from Thailand. Trace element concentrations were determined in these surface areas. The results indicate that the Fe concentrations in Mong Hsu rubies have previously been overestimated due to the presence of surface contaminations. Cu is consistently present at a few hundred ppm level in these stones and not observed in any of the four Thai rubies, while it was observed sporadically in broad-beam measurements. The depth profiling capabilities of the proton microprobe were employed for the analysis of liquid and mineral inclusions in rubies, and some preliminary results are reported.

gems, chrysoberyl, alexandrite, beryl, emerald, phenakite, Galicia, Franqueira, Spain

The Franqueira occurrence in Galicia, northwestern Spain, is described as the first documented western European deposit containing gem-quality chrysoberyl (alexandrite), emerald, and phenakite. Mineralogical, whole-rock geochemical, electron-microprobe, and fluid-inclusion studies establish its geological setting, metasomatic zonation, and probable formation conditions. Narrow granitic pegmatite dikes associated with peraluminous, synkinematic, heterogeneous two-mica granites intrude dunite and associated gabbroic rocks interpreted as remnants of overthrust ophiolitic complexes. Interaction between pegmatite-derived fluids and dunite produced phlogopitite adjacent to the dikes, tremolitite farther away, and an anthophyllite-rich zone near the remaining dunite. The dunite supplied magnesium and chromium, whereas the pegmatite supplied water, silicon, aluminium, potassium, beryllium, boron, and phosphorus. Chrysoberyl, phenakite, and emerald occur in the phlogopitite with apatite and other accessory minerals. Alexandrite crystals reach approximately 1 cm and display the characteristic change from green in daylight to reddish under incandescent illumination; colourless phenakite crystals reach 3 cm, and emerald crystals reach 30 cm. Textural relationships indicate that chrysoberyl and phenakite formed before emerald, which partially replaces both minerals and encloses their skeletal remnants. Early chrysoberyl growth may have been favoured by epitactic nucleation on residual olivine because of structural similarities between the two minerals. Emerald formation is interpreted in terms of increasing silica activity, potentially involving reaction between chrysoberyl, phenakite, and quartz until quartz was exhausted. This interpretation remains qualified because the natural system involved fluid addition, competing mineral reactions, and conditions departing from equilibrium. Microthermometry of 125 fluid inclusions in emerald and phenakite distinguishes three inclusion types and two separate hydrothermal episodes; inclusions in chrysoberyl were too small for measurement. The earlier episode involved methane-bearing and carbon-dioxide-bearing aqueous fluids, with homogenization temperatures of 318–381 °C. Their characteristics suggest contemporaneous trapping of immiscible fluids in a complex water–salt–volatile system. Combined fluid-inclusion and phase-equilibrium considerations favour gemstone formation at approximately 380–420 °C and pressures of 1–2 kbar. A later episode is recorded by secondary aqueous inclusions containing mixed salts, with homogenization temperatures of 160–265 °C, and is attributed to fluid circulation during subsequent tectonic events. Possible contributions from magmatic, metamorphic, connate, or meteoric waters require isotopic confirmation. The proposed genetic model emphasizes transfer of pegmatitic components into dunite and places Franqueira among suture-zone emerald deposits. Comparable contacts between beryllium-bearing pegmatites and chromium-rich ultramafic rocks elsewhere in the Galicia–Trás-os-Montes zone are identified as promising exploration targets.

corundum, inclusions.5, treatments.6, synthetic corundum.7, Tanzania.12, gems

gems, spinel, spinel inclusions, Tanzania

corundum, Australia.12, gems

Webb reports two unusual ruby specimens from the Inverell district of New South Wales, documenting the occurrence of ruby in an Australian gemfield better known for sapphire. The principal specimen is a faceted ruby weighing 1.41 ct, recovered by a fossicker panning in Swan Brook and subsequently retained in the private collection of its cutter. Staff of the Mineral Section of the Australian Museum examined the stone and regarded its combination of colour, quality and size as exceptional for the locality. The accompanying photograph illustrates the finished gem. Webb places this find in the context of other Australian ruby occurrences. Most Australian rubies were reported to come from the Harts Range, where the material is generally opaque and suitable for beads and cabochons. Faceting-quality ruby had also been found in the Barrington Tops area, although mining there had not been consistent. Ruby was described as a rare discovery in the sapphire fields of both Inverell and Anakie, making an attractive faceted example from either district a collector’s item. Examination of the Swan Brook stone established gemmological properties normal for ruby. It showed moderate fluorescence under long-wave ultraviolet radiation and was inert under short-wave ultraviolet radiation. A diffraction-grating spectroscope revealed a characteristic ruby spectrum, including three narrow bands in the blue region. The only inclusions observed were several partially healed fractures confined to a corner of the stone. The note also records an unusually large piece of pale ruby rough from Frazer’s Creek, Inverell, which museum staff had inspected a few weeks before examining the faceted specimen. This second ruby weighed 11.30 ct and was held by a local property owner. Together, the two specimens provide examples of the uncommon ruby component of the Inverell sapphire fields: one notable as a finished gem of unusual quality and the other for its relatively large size as rough. The report documents individual finds rather than sustained ruby production from the district.

corundum, synthetic corundum.7, Czochralski, gems

This past summer, a manufacturer of synthetic corundum shared the Czochralski-pulled synthetic ruby shown in figure 20 with staff mem-bers of the East Coast lab. The modi-fied triangular mixed cut weighed 115.56 ct and was completely trans-parent, with a strong red color that had no purple or brown components. Although natural ruby may be found in large sizes, it usually contains so many inclusions that the diaphaneity is translucent rather than transparent, One area of the pavilion of this large synthetic showed fine curved striae, but otherwise it was free of inclusions or variations in color. Because of the large size and trans-parency, a slight doubling of the pavilion facet junctions-a rare sight in corundum, which has low birefrin-gence-was visible through the table. The manufacturer of this synthetic ruby reports that he is using a new process in which growth is controlled on a scale of several inches, with the goal of producing material that is entirely free of inclusions or disrup-tions of the crystal structure (such as twinning or graining) up to sizes as large as 500+ ct.

corundum, properties.3, gems

An unusual natural cat’s-eye sapphire examined by the GIA Gem Trade Laboratory demonstrates chatoyancy produced by aligned fluid inclusions rather than the rutile needles commonly responsible for asterism in corundum. The specimen, submitted to the West Coast laboratory for identification in early 1995, was a semitransparent oval cabochon weighing 1.91 ct and measuring 13.99 × 5.19 × 2.54 mm. It had a mottled greenish blue colour and displayed a distinct cat’s-eye effect across its domed surface. Standard gemmological testing established its identity as natural sapphire. The note places the observation in the context of the relative frequency of optical phenomena in corundum: asterism is fairly common, whereas chatoyancy is encountered only rarely. In the laboratory’s experience, an apparent cat’s-eye effect in either natural or synthetic corundum usually results from the orientation chosen during cutting of an asteriated stone. When the star’s axis is positioned off-centre, only one ray extends across the cabochon dome, giving the appearance of a single eye. Microscopic examination of the submitted sapphire revealed a different explanation. The stone contained almost no needle-like rutile crystals, prompting investigation of other internal features capable of producing the observed effect. Instead, it contained numerous approximately planar, mutually parallel, liquid-filled inclusions described as fingerprints. The accompanying photomicrograph illustrates these as parallel liquid-filled channels. Light scattering from this aligned inclusion system was identified as the cause of chatoyancy. Examination also revealed diffuse colour halos surrounding crystal inclusions, which were interpreted as evidence that the sapphire had undergone heat treatment. The report therefore documents both an uncommon inclusion-related optical effect and microscopic evidence of treatment in the same natural stone. Its principal finding is that a distinct eye in sapphire can originate from parallel fluid-filled structures, providing an alternative to the more usual explanation involving the cutting orientation of a star-bearing corundum cabochon.