ruby, sapphire, corundum, Burma.12, inclusions.5, quality.10, gems

Reviews the history, geology and ruby mining of Burma’s Mogok Stone Tract, drawing on published literature and accounts from earlier visitors. Describes ruby-bearing marbles and the secondary deposits produced by their weathering, together with traditional and mechanized mining methods. Summarizes the gemological properties and characteristic inclusions of Mogok rubies and discusses notable historic stones.

corundum, ruby, sapphire, tsavorite, garnet, tourmaline, tanzanite, Kenya.12, Tanzania.12, East Africa, geology.11, gems, andalusite, apatite, beryl, diamond, diopside, euclase, feldspar, fluorite, iolite, idocrase, hematite, kyanite, lazulite, obsidian, olivine, peridot, opal, quartz, rhodonite, rutile, sphalerite, sphene, turquoise, zircon, gem localites, general gemology, pearls, emerald, Brazil, Colombia

This substantial volume collects the papers presented at the Gemological Institute of America’s 1982 International Gemological Symposium, bringing together research and practical observations from gemologists, mineralogists, scientists, dealers, and other specialists. The proceedings provide a broad survey of gemology at the beginning of the 1980s, covering natural and synthetic gemstones, gemstone identification, inclusions and microscopy, treatments and their detection, diamond research, colored stones, pearls, mining and geographic sources, instrumentation, cutting, and developments affecting the gem trade. Particular attention is given to the rapidly developing problems created by synthetic and treated materials and to the analytical and observational techniques required to distinguish them from their natural counterparts. Individual contributions also document important gem deposits and varieties and demonstrate the increasing application of mineralogical, chemical, spectroscopic, and microscopic methods to practical gem identification. As a whole, the proceedings provide a useful snapshot of the state of gemological research and laboratory practice in the early 1980s and record a period in which treatment detection, synthetic gem identification, and increasingly sophisticated scientific instrumentation were becoming central concerns of professional gemology.

gems, corundum, inclusions.5, Burma.12, synthetic corundum.7

corundum, inclusions.5, gems

corundum, synthetic ruby, synthetic corundum.7, flux, gems

A new synthetic ruby, developed by Professor P.0. Knischka using a method of gradient techniqut (not yet disclosed) through supercooling and saturation, has distinct crystallographic and gemological characteristics. Particularly notable are the great number of faces on the finished crystals and the identifying inclusions.

corundum, inclusions.5, photomicrography, gems

The shadowing technique employs an opaque, black, nonreflecting light shield that is inserted gradually into the transmitted light path of a gemological microscope between the subject and the light source. Shadowing takes advantage of even very slight differences in refractive index between a gemstone host and its inclusions, casting shadows in certain areas of an inclusion scene and passing light to the microscope objectives in other areas of the image. Thus, shadowing produces a three-dimensionality that seems to lift the inclusion from its surroundings, revealing it in vivid contrast against the now-subdued background of the host. This increase in contrast adds much greater detail to growth zones, included crystals, color zones, and the like, thus greatly aiding the gemologist in his work with the microscope.

corundum, synthetic corundum.7, gems

Gübelin discusses the identification of the newer flux-grown synthetic rubies, particularly those produced by Kashan, Chatham and Knischka. He emphasizes that traditional criteria for separating natural from synthetic ruby were becoming inadequate because the newer products could contain inclusions closely resembling those of natural stones. Particular attention is given to microscopic features, illustrated by 41 color photographs, including feather-, veil- and fingerprint-like inclusions, fine needle-like inclusions, and growth structures. Cryolite (Na₃AlF₆) was identified by X-ray analysis among the inclusions in flux-grown material. Knischka synthetics are described as containing rutile-like needles, negative crystals, two-phase inclusions and metallic platelets. Gübelin also considers ultraviolet fluorescence, phosphorescence and absorption spectra as supplementary identification criteria.

corundum, synthetic corundum.7, color.4, gems

Develops a quantitative ultraviolet–visible spectrophotometric method for distinguishing natural from synthetic rubies, refining earlier short-wave ultraviolet immersion contact photography. The work addresses the increasing difficulty of identification by inclusions as synthetic growth techniques improve, particularly where low-iron natural rubies resemble iron-bearing synthetics. Approximately 250 absorption spectra were obtained from natural rubies representing Myanmar, Sri Lanka, Kenya, Tanzania, Thailand, and several additional sources, together with Verneuil, Chatham, Kashan, and Knischka synthetic material. Measurements used a double-beam spectrophotometer, with attention to sample positioning and the ordinary and extraordinary spectral components. Rather than relying on overall ultraviolet transparency, the method evaluates the wavelength of the ultraviolet absorption minimum and the centre position and width of its profile measured 0.5 absorbance units above that minimum. Plotting minimum wavelength against the ratio of profile centre wavelength to width separated the natural and synthetic populations examined. Natural rubies generally exhibited narrower minima at longer wavelengths, whereas synthetic rubies occupied broader, shorter-wavelength fields. No overlap was observed, although limited overlap was considered theoretically possible near adjoining population boundaries. Myanmar rubies and some Chatham, Kashan, and Knischka synthetics presented the closest approaches between the two groups. Additional criteria included ultraviolet absorption relative to the blue-region minimum, profiles at other absorbance levels, the onset of strong ultraviolet absorption, and subsidiary bands. Spectral differences were interpreted chiefly in terms of chromium, iron, and titanium. A band near 266 nm, particularly prominent in titanium-rich Kashan synthetics, was tentatively attributed to Ti³⁺, with further chemical evidence required to confirm the assignment. Elevated titanium was also associated with abnormal dichroism in Kashan rubies. Complementary fluorescence measurements showed that emission spectra were unsuitable for reliable separation, while excitation spectra distinguished only more extreme compositional groups because several natural and synthetic populations overlapped. Qualitative energy-dispersive X-ray fluorescence analyses of eight representative stones supported the inferred trace-element differences. Heat-treated natural rubies reportedly retained absorption characteristics comparable to untreated material from the same locality. A significant limitation concerned natural ruby nuclei with synthetic overgrowths, which yielded intermediate parameters dependent on the relative optical contributions of core and overgrowth. Bosshart concludes that numerical evaluation of ultraviolet absorption minima provided reliable identification within the investigated collection, including stones whose colour and fluorescence behaviour were otherwise ambiguous, while recognising the need for supplementary evidence in special cases.

corundum, sapphire, treatments.6, synthetic corundum.7, synthetic sapphire, inclusions.5, gems

Heat treatment and impurity diffusion are reviewed as distinct but sometimes combined processes that alter the colour, transparency, asterism and internal features of natural and synthetic corundum. Drawing on laboratory observations, published technical information and patents, the authors distinguish treatments capable of affecting an entire stone from those whose effects are restricted to a thin surface layer. Heating alone can develop asterism, dissolve rutile silk, strengthen or diminish blue colour under reducing or oxidising conditions, and reduce curved growth banding and strain in Verneuil synthetics. Surface diffusion of titanium, iron, chromium or other impurities can introduce asterism or colour. Commercial treatment of milky Sri Lankan geuda to produce transparent blue sapphire illustrates the combined removal of silk and development of colour. Features associated with heated natural blue sapphires include dull, chalky green fluorescence, frequently absent iron absorption near 450 nm, stress fractures around inclusions, altered rutile-related colour zoning, cross-hatched colour banding and surface damage incompletely removed during repolishing. These observations can support recognition of treatment, although conclusive identification is not possible in every case. Yellow to orange sapphires present additional problems because their colour may result from heating or irradiation; some treated examples resisted sunlight exposure, whereas irradiation-induced colour is described as unstable. Diffusion-treated faceted stones and cabochons are most readily examined by immersion in methylene iodide, which reveals uneven surface colour, colour concentrations at facet junctions, areas depleted by polishing, and colour extending inward along pits and fractures. Mountings can conceal these features, and an apparently attractive, even face-up colour does not exclude diffusion. Because the added colour is shallow, recutting or repolishing may remove it. Recently examined star sapphires combined unusually sharp stars and extremely fine needles with evidence of colour diffusion, indicating induced asterism as well as colour enhancement. Treatment of Verneuil synthetic sapphires and rubies introduces a further identification difficulty: curved banding and strain may become less conspicuous, while induced fingerprint-like inclusions can imitate features commonly associated with natural stones. Gas bubbles, residual growth features, fluorescence and careful optical examination remain relevant to their recognition; fingerprint inclusions alone cannot establish natural origin. The authors emphasise that commercial treatment conditions were incompletely documented and that corundum from different localities may respond differently. They reject a simple analogy with heating aquamarine, arguing that the diversity and potential detectability of corundum treatments require broader diagnostic awareness. Disclosure is considered essential for identifiable processes that introduce artificial surface colour, asterism or misleading internal features.

corundum, inclusions.5, Thailand.12, gems

Although the general principles of photomicrography are easily learned and applied, high-quality photomicrography is an art that is mastered only with time and great patience. The microscope must be kept scrupulously clean, and the effects of light on the subject inclusion must be fully understood in order to determine what method(s) of illumination will yield the most useful photographic image. Specialized techniques that can save film and time, while producing top-quality photomicrographs, are usually learned only through long hours of experience. This article discusses some of these techniques, such as the importance of a properly prepared microscope and photographic subject, as well as the control of vibrations and exposure time. In addition, the various methods of illumination that are adaptable to a standard binocular gemological microscope are introduced.