alexandrite, Brazil, gems

Gübelin reports on the newly discovered Hematita alexandrite deposit in Minas Gerais, Brazil, discovered in 1987 and notable for producing unusually fine and relatively clean material. He describes the appearance and gemological characteristics of the alexandrite, including its pronounced color change from greenish hues in daylight to reddish or purplish red under incandescent illumination. Particular attention is given to internal features useful for identifying the Brazilian material. Among the characteristic mineral inclusions subsequently identified in Hematita alexandrite are fluorite and apatite, together with mica and fluid inclusions. The discovery was significant because Hematita rapidly became an important source of fine alexandrite at a time when supplies from the traditional Russian and Sri Lankan sources were limited.

corundum, chemistry.2, gems

Harder and Schneider investigate experimentally how cooling rate controls whether Ti-bearing corundum develops asterism or a cornflower-blue color. They relate the two effects to the behavior of titanium during cooling: under conditions permitting exsolution, titanium is precipitated as oriented rutile needles, producing the silk responsible for asterism; under different cooling conditions, titanium remains incorporated in the corundum lattice, where interaction between iron and titanium produces blue coloration. Thus, corundum of essentially comparable composition can develop either the oriented rutile inclusions required for a star effect or the transparent blue appearance characteristic of sapphire, depending upon its thermal history. The paper emphasizes cooling rate as a decisive factor governing the distribution of titanium between the corundum lattice and exsolved rutile.

inclusions, gems

Discusses the formation of mineral inclusions in gemstones and their relationship to the growth history of the host crystal. Reproduces a chapter from the author’s Photoatlas of Inclusions in Gemstones.

corundum, inclusions.5, gems

Describes thin diaspore lamellae in blue-grey, barrel-shaped corundum crystals up to 5 cm long from Mato Grosso, Brazil. Under slight mechanical pressure, the crystals split readily along gently curved surfaces lacking a consistent crystallographic orientation. A thin mineral layer exposed on these surfaces has a lower refractive index than corundum and was identified as diaspore. The unusual splitting is attributed to these lamellae, interpreted as products of retrograde metamorphism.

peridot, inclusions, gems

Gübelin gives a brief illustrated account of inclusions characteristic of peridot. He describes the internal features encountered in gem-quality olivine, emphasizing mineral inclusions and their diagnostic appearance under the microscope. Typical features of peridot include chromite and other spinel-group crystals, commonly surrounded by stress fractures or disc-like halos, as well as negative crystals and partially healed fractures. Such inclusions are useful both for identifying natural peridot and for interpreting aspects of its geological formation.

beryl, synthetic beryl, emerald, synthetic corundum, gems

Gübelin reviews the diagnostic properties of several recently introduced synthetic gemstones and the problems they pose for separation from their natural counterparts. Emphasis is placed on the combination of conventional gemological properties with microscopic examination of inclusions and growth structures, supplemented where necessary by spectroscopic and other analytical methods. The article discusses characteristic features of newer synthetic materials, particularly synthetic corundum and emerald, and shows that physical constants alone may be insufficient for identification as manufacturing techniques increasingly reproduce the appearance and properties of natural stones. Gübelin stresses the diagnostic importance of flux residues, metallic inclusions, growth zoning and other internal features associated with particular synthesis processes. The paper summarizes practical criteria by which these newer products can be recognized in gemological testing.

inclusions, gems

Weibel provides a concise survey of the principal gemstones with particular emphasis on their mineral inclusions and what these reveal about gemstone formation. Rather than being solely an inclusion atlas, the book introduces the major gem species and relates their characteristic internal minerals and structures to their geological genesis. Numerous photomicrographs illustrate diagnostic inclusions and demonstrate their value for understanding the conditions under which their host gemstones crystallized. The volume was prepared with the assistance of Eduard J. Gübelin, who also supplied some of the photographs. It contains 83 color and 23 black-and-white illustrations. A contemporary Journal of Gemmology review specifically praises its treatment of inclusions and the accompanying explanations of their formation.

gems, corundum, synthetic corundum.7, Ramaura, flux, YAAG, synthetic garnet

Gübelin describes two recently introduced artificial gem materials: Ramaura flux-grown synthetic ruby and yttrium aluminium gallium garnet (YAGG), a green artificial material used as an imitation of tsavorite. For Ramaura ruby, he discusses manufacture by flux growth at about 1250°C, the use of chromium as the coloring agent, and the effects of flux constituents and growth conditions on crystal form. Because its conventional physical properties closely resemble those of natural ruby, identification relies heavily on microscopic examination; numerous characteristic inclusions are illustrated, while UV-visible spectroscopy is reported to offer no certain separation. The second material, YAGG, differs markedly from tsavorite in its physical properties, with refractive index around 1.90 and specific gravity 5.06–5.08. Gübelin describes its composition and characteristic yellowish flux inclusions and notes that, despite its garnet structure, it is not a silicate. The article contains 42 photographs and illustrations.

corundum, synthetic corundum.7, gems

Gübelin examines the identification of newly introduced Chatham flux-grown blue and orange synthetic sapphires and of natural sapphires whose color has been altered by heat treatment or surface diffusion. The physical properties of the Chatham synthetics differ little from those of natural sapphire, making microscopic examination particularly important. Blue Chatham sapphires may show irregular blue, yellow, and colorless zoning, straight or angular growth structures, polysynthetic twinning, coarse or delicate flux residues forming feathers, veils, nets, lace-like patterns, and misleading “fingerprints,” together with black platinum platelets, rods, splinters, and needles that reflect with a characteristic silvery metallic lustre; X-ray analysis confirmed the metallic inclusions as platinum. Some blue specimens also fluoresce intense pale green under long-wave ultraviolet radiation. Chatham orange sapphires range from brownish orange through orange-red to pinkish-orange “padparadschah” colors and likewise contain flux residues and platinum inclusions that provide important evidence of synthetic origin; ultraviolet absorption and electron-spin-resonance measurements are also investigated as possible means of separation from natural stones. Gübelin then discusses heat-treated natural sapphires, in which high temperatures may produce pockmarked surfaces, altered or melted mineral inclusions, circular tension fractures with lace-like rims, enhanced cloudy or phantom-like zoning, and fine exsolved particles producing turbidity. Diffusion-treated sapphires may additionally show strongly colored surface-reaching fissures and inclusions, irregular surface coloration, and concentrations of color along facet junctions and the girdle; immersion in a highly refractive liquid makes these features particularly conspicuous. Gübelin emphasizes that no single characteristic is universally diagnostic and recommends careful comparison of macroscopic appearance, physical properties, ultraviolet response, spectra, and especially microscopic inclusions when distinguishing natural, synthetic, heat-treated, and diffusion-treated sapphires.

inclusions, gems

Gübelin discusses mineral inclusions in colored gemstones from the standpoint of their genesis and the information they provide about the geological environment in which their host crystals formed. He distinguishes inclusions according to their relationship to the growth history of the host—pre-existing minerals incorporated during growth, minerals formed contemporaneously with the host, and minerals produced by later alteration or exsolution. Particular attention is given to the morphology, orientation and mineralogical identity of inclusions and to mineral associations characteristic of different geological environments. Such inclusion assemblages can provide evidence for the conditions and sequence of gemstone formation and, in favorable cases, assist in determining geographic origin. The article represents Gübelin’s broader approach of treating inclusions not merely as identification features but as records of gemstone genesis.