corundum, synthetic corundum.7, gems

Gübelin and Knischka describe a new gem-quality synthetic ruby produced by Paul O. Knischka using a proprietary “gradient technique” involving supersaturation and controlled cooling. The resulting crystals have an unusual nearly isometric or pseudocubic habit and exceptionally numerous well-developed, undamaged crystal faces, with examples possessing 32 or 38 faces. The authors compare several types of Knischka synthetic ruby with natural and Verneuil synthetic ruby, examining crystal morphology, trace-element chemistry, optical and gemological properties, fluorescence, and inclusions. In color and general appearance the material closely resembles natural ruby, although it tends toward a more violet red; refractive index, birefringence, and other standard properties offer little assistance in separation. Microscopic features include folded or feather-like inclusions, color zoning, two-phase inclusions, negative crystals, and occasional metallic platelets, some of which can resemble features in natural stones. The study emphasizes the increasing difficulty of distinguishing high-quality synthetic ruby from natural material and the potential diagnostic value of inclusions and trace-element composition.

tsavorite, grossular, Kenya, gems

Describes vanadium-bearing green grossular from the Lualenyi mine in Kenya’s Taita district, comparing its occurrence with material from Tanzania’s Lalatema Hills and Merelani, where grossular accompanies blue zoisite. The green colour is attributed to vanadium, with approximately 3% V₂O₃ reported. Internal features include probable byssolite inclusions and trails of minute liquid droplets.

beryl, emerald, Colombia, gems

Barriga Villalba and Barriga del Diestro present a monographic study of Colombian emerald, describing its principal characteristics and properties. The work considers emerald as a mineral and gemstone, with attention to its physical and optical properties, characteristic color and inclusions, and the features associated with Colombian material. It also discusses the geological occurrence and principal emerald-producing areas of Colombia, particularly the deposits of the Eastern Cordillera, and addresses aspects of mining and the evaluation of emeralds. The volume provides a contemporary Colombian account of the country’s most important gemstone and its distinguishing gemological and geological characteristics.

inclusions, gems

inclusions, gems

First comprehensive article on the determination of inclusions in gemstones by means of the electron microprobe. Techniques are described and results explained. Fine color illustrations. Essentially the same article appeared in Journal of Gemmology, 11(5), 1969, pp. 149–192, with 80 color photographs, 3 figures, and 49 references.

jade, jadeite, fei cui, nephrite, Wyoming, Burma, inlusions, gems

ekanite, gems

Gübelin gives a detailed gemological account of ekanite, a newly recognized metamict gem mineral from Ceylon, named for F. L. D. Ekanayake, who first recognized its unusual character. The material occurs in the gem gravels near Ellawala, northeast of Ratnapura, and is typically greenish-brown to yellowish-green, transparent to translucent, and sometimes displays chatoyancy or asterism caused by oriented clouds of minute inclusions. Gübelin describes its gemological properties, including its singly refractive character, refractive index of about 1.595, specific gravity of about 3.28, and weak absorption bands, together with microscopic features and inclusions. Chemical investigation showed calcium, thorium, silicon, aluminium, and lead, while measurements demonstrated significant radioactivity associated particularly with thorium. The destruction of the original crystal lattice through radioactive decay accounts for its metamict condition. Gübelin also discusses its probable original crystalline state, chemical composition, and age as estimated by thorium-lead dating, establishing ekanite as an unusual new addition to the gem minerals of Ceylon.

corundum, synthetic corundum.7, gems

Describes ruby specimens consisting largely of natural cores covered by thin synthetic layers, reportedly grown hydrothermally. Their physical constants match natural ruby, and inclusions within the cores can make identification misleading. Minute gas bubbles in the coating and a sharp boundary between core and overgrowth reveal their composite nature; the observed features suggest growth from a melt rather than a hydrothermal process.

corundum, Brazil.12, gems

Describes sapphire crystals from the Coxim River area of Mato Grosso, Brazil, noting unusual rounded inclusions resembling the gas bubbles commonly associated with synthetic corundum.

phase-contrast microscopy, gems

Explains the application of Zernike’s phase contrast microscopy to reveal subtle internal features in gemstones. By converting differences in optical path into visible brightness contrasts, the method enhances thin, transparent inclusions and structures whose refractive indices closely resemble those of their hosts. Specially adapted equipment permits examination within gemstones, revealing exsolution, alteration and healed fractures that are difficult to distinguish under ordinary illumination.