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.

inclusions, gems

Discusses inclusions in gemstones.

corundum, ruby, sapphire, Sri Lanka.12, gems

A concise account of the crystalline rocks of Ceylon, intended to summarize their character and classification as then understood. Coomaraswamy discusses the terminology of “gneiss” and “granulite,” preferring the latter for many Ceylonese rocks, and regards them as belonging to the Charnockite Series described from southern India, the two regions forming a closely related petrographical province. He describes the principal rock types and their mineral banding, together with the crystalline limestones and rocks intermediate between limestone and granulite. The latter may contain diopside, phlogopite, amphibole, scapolite, spinel, sphene, calcite and, more rarely, serendibite, and may occur at limestone–granulite contacts, as inclusions, dykes or bands; some also yield mica of commercial value. Coomaraswamy argues that, whatever their ultimate origin, the crystalline limestones were essentially contemporaneous with the granulites and crystallized under broadly comparable high-temperature conditions. He also distinguishes the wollastonite-bearing Galle group and discusses the structural geology of the crystalline series, particularly foliation, folding and the influence of rock structure on the development of parallel strike valleys such as those around Hatton. The paper is an important early synthesis of Ceylon’s Precambrian crystalline geology and provides geological context for the mineral and gem deposits associated with these rocks.