corundum, ruby, sapphire, Kenya.12, Tanzania.12, East Africa, geology.11, gems

An illustrated survey of East African gemstones, emphasizing the deposits of Kenya and Tanzania and combining regional geology with accounts of discovery, exploration, mining, and gemological properties. Following an introduction to the geological setting, Keller discusses diamonds, ruby and sapphire, emerald and alexandrite, tanzanite, garnets, tourmaline, and green opal, with a concluding chapter on less prominent gem materials. Individual accounts place the deposits within their geological environments while recording their development and the people involved. Particular subjects include tanzanite from Merelani and prase opal from Haneti Hill, Tanzania. Photographs of rough and cut stones, inclusions, mining operations, and historical scenes complement locality maps, including some previously unpublished examples. Contributors include E.J. Gübelin, Harold and Erica Van Pelt, and J.I. Koivula. Appendices list 177 noteworthy gem localities with geographical coordinates and document East African gemstones in the collections of the Los Angeles County Natural History Museum and the Smithsonian Institution. References accompanying each chapter provide access to the region’s geological and gemological literature. The volume offers a substantial record of East African gem deposits and industry as understood at the beginning of the 1990s.

corundum, chemistry.2, inclusions.5, geology.11, Australia.12, gems

corundum, treatments.6, diffusion-treated, gems

The Summer 1990 issue of Gems & Gemology carried a comprehensive article on diffusion-treated sapphires. Among the materials examined were stones marketed by the firm Gem Source of Las Vegas, Nevada, and Bangkok, Thailand. At that time, Jeffery Bergman of Gem Source stated that he used Sri Lankan sapphires as starting material. Recently, Mr. Bergman shared information relating to experimental diffusion treatment of sapphires from another locality: Montana. He also donated eight of these stones, ranging from 0.53 to 1.18 ct, to GIA for study. According to Mr. Bergman, approximately 50% of the Montana sapphires they have diffusion treated to produce a blue color have also developed unwanted yellowish orange zones. Examination of the eight sample stones with magnification readily revealed the yellowish orange zones described by Mr. Bergman; also noted in some of the stones were disk-like areas of dark blue surrounding dark crystalline inclusions (figure 12). We believe that the yellowish orange areas are related to zoned growth and reflect a higher iron impurity content, while the disk-like dark blue areas are caused by internal diffusion—a cannibalization of both iron and titanium from possibly iron-rich rutile and/or ilmenite inclusions. Both of these unanticipated color areas no doubt result from the high temperatures used in the diffusion-treatment process.

corundum, sapphire, USA.12, Montana.12, inclusions.5, Dry Cottonwood Creek, gems

gems, inclusions, beryl, emerald, Australia, Norway

The second installment of Schwarz’s systematic study of the chemical properties of emeralds from different geographic sources, dealing with material from Australia and Norway. Norwegian specimens are from Eidsvoll near Lake Mjøsa, while the Australian material represents several occurrences, including Pilangoora, Wodgina, Calvert White Quartz Hill, Warda Warra, Melville, Torrington, Emmaville and Menzies. Chemical analyses are used to compare major and trace-element compositions and to evaluate variations among deposits and against previously studied Brazilian emeralds. Schwarz notes, among other distinctions, generally lower magnesium contents than in Brazilian material. The paper demonstrates the usefulness of chemical composition, considered together with gemological properties, for characterizing emeralds and assessing their geographic origin.

corundum, treatments.6, agate, chalcedony, dying, gems

corundum, inclusions.5, Kenya.12, Australia.12, Sri Lanka.12, treatments.6, gems

corundum, synthetic corundum.7, synthetic sapphire, triangular bubbles, Verneuil, gems

gems, garnet, pyrope-almandine, rhodolite, star garnet, star rhodolite, Tanzania

This article describes the first reported occurrence of asteriated rhodolite garnet from Tanzania and details the specimen’s gemmological properties. Acicular rutile crystals are well-known inclusions in almandine garnets (Webster, 1983; Gübelin and Koivula, 1986; Liddicoat, 1987), and when fashioned en cabochon, stones with a relatively dense and even distribution of these needle-like inclusions may exhibit asterism, typically as a four- or six-rayed star. Sources of star almandine garnet include India (Webster, 1983; Rohrbach, 1986) and the Emerald Creek area of Idaho, USA (Zeitner, 1986; Rohrbach, 1986). Rhodolite, a reddish-purple to purple garnet composed of a mixture of almandine and pyrope, was first described from North Carolina, USA (Hidden and Pratt, 1898), and is also found in Sri Lanka (Webster, 1983), a major commercial source (Federman, 1984). In recent decades, African localities such as Zimbabwe (Campbell, 1972), Madagascar (Campbell, 1973), and Tanzania (Phillips, 1962; Sinkankas, 1968; Sarofim, 1970; Pough, 1973) have also produced rhodolite, with Tanzania now serving as an important commercial source (Federman, 1984). Like almandine, rhodolite from several sources may contain acicular rutile inclusions (Campbell, 1972; Webster, 1983), though few references have noted these inclusions producing asterism in rhodolite (Rohrbach, 1986). The authors recently examined a rhodolite garnet cabochon that displayed a four- or six-rayed star, depending on the viewing angle. The stone was cut from rough purchased and fashioned by Mr. Barton Curren of Glyptic Illusions, Topanga, California, USA, from a Tanzanian parcel reportedly originating from the Kangala mine. According to Curren, the rough was relatively inexpensive due to its higher inclusion content compared to other Tanzanian rhodolite he had previously acquired.

India.12, Kashmir.12, sapphire, corundum, gems

Examines the geological history preserved in mineral and fluid inclusions in Kashmir sapphires from the Sumcham area of the Zanskar Himalaya. The sapphires occur in quartz-free pegmatites near contacts between ultramafic rocks and amphibolites. Mineral chemistry, Raman spectroscopy and microthermometry reveal three successive growth zones formed under declining pressures and temperatures, spanning approximately 5.6–3.1 kbar and 700–500°C. Predominantly carbon dioxide fluid inclusions record repeated pressure fluctuations, fracturing and fluid movement during crystal growth. The authors propose initial crystallization from partial melts, followed by growth involving residual magmatic and metamorphic fluids, and a final hydrothermal stage. Mobilization of iron and titanium from surrounding amphibolites under suitable redox conditions produced the intense blue colour. The inclusions document sapphire formation at substantial crustal depths during active deformation and uplift of the Himalaya.