corundum, Burma.12, gems

Combines a travel narrative through Myanmar with an illustrated account of Mogok’s gemstones, mining and trade. Introduces the country’s history, geography and culture before examining Mogok’s geological setting, ruby and sapphire deposits, and other gem minerals. Covers corundum properties, inclusions, treatments, quality assessment, pricing, historic stones and trade routes, concluding with cutting and jewelry manufacture. Photographs, maps and a glossary support the text.

gems, beryl, emerald, Egypt, Sikait, Austria, Habachtal, archaeology, geology, ancient gem trade, chemistry, oxygen isotopes

Examines the ancient exploitation and trade of emeralds from Austria and Upper Egypt through oxygen-isotope analysis and inclusion studies of archaeological and historical specimens. Giuliani and co-authors compare emeralds from the Habachtal deposit in Austria, the Ural Mountains, and Egyptian deposits at Zabara, Sikait and Umm Kabo with emeralds from Gallo-Roman and Egyptian jewellery. Oxygen-isotope ratios, supplemented where necessary by the petrography of fluid inclusions, provide distinctive signatures capable of separating the principal ancient sources. An emerald from a third-century AD earring in the Vaise Treasure at Lyon yields an isotopic composition characteristic of Habachtal, demonstrating that Austrian emeralds were being exploited by the Gallo-Roman period. Four emeralds from an Egyptian necklace in the Muséum National d’Histoire Naturelle have isotope values consistent with Upper Egyptian material, while nineteenth-century specimens collected from the Egyptian mines confirm the restricted isotopic range of those deposits. The study also reviews earlier analyses indicating Austrian origins for emeralds in the Visigothic Guarrazar crown and the medieval French crown, an Egyptian origin for other Roman-period stones, and a Pakistani source for an emerald from a Gallo-Roman earring at Méribel. The results support the historical reality of active emerald mining in both Upper Egypt and Austria during Roman times and demonstrate that emeralds from more distant Asian sources also reached the Roman world through long-distance trade routes.

gems, beryl, emerald, Colombia, geology, emerald formation

Emerald mineralisation in Colombia occurs in two separate zones along the margins of the Eastern Cordillera, approximately 80 km apart, with the western zone dated to around 35 million years ago and the eastern zone about 30 million years older. Crush-leach analysis of fluid inclusions in minerals such as emerald, quartz, calcite, dolomite, and fluorite from both zones reveals two distinct brine types associated with emerald formation, with many samples showing intermediate compositions. Fluid 1, primarily found in emerald-hosted inclusions, is dominated by NaCl and displays high Cl:Br ratios, indicating salinity from halite dissolution, likely sourced from local salt beds. Fluid 2, notably present in quartz-hosted inclusions, has similar salinity but contains lower Na and higher levels of Ca, K, Fe, and other cations, with lower Cl:Br ratios more typical of formation waters, though it also likely derives salinity in part from halite. Linear trends in bivariate plots of cation concentrations, regardless of host mineral or locality, confirm that mixing between the two fluids occurred. The presence of these same fluid types in both zones, despite differences in age and geography, suggests that fluid chemistry was controlled by similar interactions with evaporites and black shales in both cases. It is proposed that beryllium was transported as Be–F complexes in the NaCl-rich fluid and that emerald precipitated upon mixing with the calcic brine, which also triggered the precipitation of fluorite and parisite.

corundum, treatments.6, gems

One of the important issues at Tucson this year was the nature of the “fillings” in heat-treated rubies. A major concern was the potential misidentification of a natural surface-reaching inclusion as foreign material (see, e.g., J.L. Emmett, “Fluxes and the heat treatment of ruby and sapphire,” Gems & Gemology, Fall 1999, pp. 90–92). Kenneth Scarratt provided the following information. Since 1984, laboratories and gemologists have identified substances in cavities within ruby and sapphire as “glass” based on visual indicators such as surface luster, bubbles, and evidence of devitrification (see, e.g., R.R. Harding and K. Scarratt, “Glass infilling of cavities in natural ruby,” Journal of Gemmology, October 1984, pp. 293–297; R.E. Kane, “Natural rubies with glass-filled cavities,” Gems & Gemology, Winter 1984, pp. 187–199). Sophisticated techniques have not been used routinely, owing to the high cost of advanced testing and, until recently, the limited quantity of glass-filled rubies. In Bangkok, it has become common practice to use hydrofluoric acid—an aggressive acid that easily dissolves silicates and must be handled with extreme caution—to remove glass residues from rubies after heat treatment. This process is carried out either by the treating company or gem laboratories as an added client service. Typically, rubies are immersed in hydrofluoric acid for a few seconds or minutes to remove glass from surface-reaching cavities. However, in recent years, several instances have emerged in which materials that appeared visually to be glass could not be removed with hydrofluoric acid. These materials shared all the visual characteristics of glass fillings, but their resistance to acid treatment cast doubt on their identity. Consequently, the AGTA Gemological Testing Center in New York has adopted a policy of recording Raman spectra for all “glass-like” substances in ruby cavities. This has been enabled by newer, simplified laser Raman microspectrometers. During implementation, the Center found several fillings that looked like glass under magnification but were not silicate glass. In one case, a cavity featured a plateau of a transparent, slightly undulating, glassy material beneath the ruby’s surface. Raman spectroscopy revealed it was actually corundum with a small area of anatase (a titanium oxide mineral). Both corundum and anatase exhibit sharp peaks at characteristic wavenumbers in their Raman spectra, while true glass does not. This example underscores the importance of caution and proper analytical techniques in identifying substances within ruby cavities. Visual cues alone are insufficient for definitive identification.

corundum, geology.11, sources.12, Australia.12, gems

Sapphires, predominantly medium to dark blue but also in other colors (e.g., green and parti-color), have been mined in eastern Australia for more than 100 years. Most have come from two major fields: Anakie in Queensland and New England in New South Wales. The sapphires are alluvial, derived from the erosion of alkali volcanic rocks (basalt lavas, pyroclastics, and volcaniclastics) of Tertiary and Quaternary age. They are similar in many ways (including gemological properties) to sapphires from Thailand, Cambodia, Nigeria, and China, which also are associated with alkali volcanic rocks of similar ages. The gem properties of Australian sapphires are typical for sapphire, with a broad range of refractive index (nε = 1.761–1.765, nω = 1.769–1.774) and specific gravity (3.97–4.02) values. The diaphaneity varies from transparent to opaque. Most stones exhibit a strong 450 nm iron-related absorption band with a spectroscope, and most are inert to both long- and short-wave UV radiation. Australian sapphires are characterized by their inclusions (e.g., pyrochlore, ilmenite, and spinel), chemistry (high iron content), and strong growth zoning and color banding. Nearly all are heat treated to improve clarity and color. The average size of faceted Australian sapphires is under 2 ct. The largest recorded rough sapphire, which weighed 4,180 ct, was carved into a 3,294 ct bust of Dr. Martin Luther King, Jr. Although numerous alluvial occurrences of ruby have been reported in Australia, gem-quality material is rare, and none of these deposits has ever been economic. The first discovery with significant potential was within gneiss in the Harts Range (Northern Territory). However, mining ceased after only minor production of mostly low-grade cabochon material. Sapphire mining in Australia was minor and sporadic from the 1880s until the early 1960s, except for a few favorable periods—such as immediately preceding World War I, when there was strong demand from Germany and Imperial Russia. Demand for sapphires increased in the 1960s and, with the arrival of buyers from Thailand, there was unprecedented activity in the Anakie and New England fields. Mining reached its peak in the mid-1970s, making Australia a major world producer. Since then, the Australian sapphire industry has declined because of the depletion of known resources, increased costs, and difficult land access. The situation has been further exacerbated by the discovery in recent years of important deposits in countries that have lower mining costs, mainly Madagascar, Tanzania, and Nigeria. Since 1998, sapphire mining in Australia has effectively ceased, and the industry faces an uncertain future.

corundum, treatments.6, Burma.12, gems

THE DISCOVERY OF EXTENSIVE RUBY deposits in the Mong Hsu area of Myanmar (Burma) has provided the marketplace with a much-needed source of fine ruby in smaller sizes (see, e.g., Hlaing, 1991; Henn and Bank, 1993; Kammerling et al., 1994). Much of the ruby from this location is characterized by blue or blue-black cores which generally are removed by heat treatment (Peretti et al., 1995). The heat treatment is straight-forward: Temperatures of 900°C or above in an air atmosphere are usually sufficient to accomplish the desired change in color. When substantial quantities of Mong Hsu ruby began arriving at gemological lab-oratories worldwide, new features were recognized that were soon identified as residual flux inclusions (Peretti, 1993; Milisenda and Henn, 1994). These observations were mentioned in grading reports (Hänni et al., 1998), and have initiated a serious debate on the use of flux in heat treatment and, in particular, what the presence of large amounts of flux means in terms of the quality and durability of the original ruby.

gems, beryl, emerald, Colombia, inclusions

Schwarz reviews the diagnostic value of solid and fluid inclusions in distinguishing natural from synthetic emeralds and assessing the provenance of natural stones. He separates genetic classifications, based on inclusion formation relative to crystal growth, from descriptive classifications better suited to routine gemmological identification. Inclusions reflect host-rock mineralogy, mineralizing fluids, and growth conditions. Natural emeralds commonly contain mica, amphibole, carbonate, and other minerals; distinctive assemblages can indicate particular geological environments or localities, whereas widespread inclusions such as mica generally cannot establish geographical origin. Synthetic emeralds may contain flux residues, metallic particles derived from growth vessels, phenakite, and other crystalline inclusions associated with flux or hydrothermal production. Fluid inclusions vary in cavity shape, distribution, and phase composition. Schwarz emphasizes that apparently characteristic features require caution: three-phase inclusions resembling those of Colombian emeralds also occur in Nigerian material, while hydrothermal synthetics may display fluid inclusions and healed fissures resembling natural examples. Flux-grown emeralds instead contain solidified flux, sometimes accompanied by shrinkage cavities. Illustrated examples demonstrate both the importance of inclusion examination and the limits of identifying origin from individual features.

gems, beryl, emerald, Zimbabwe, Rhodesia, Sandawana

Zimbabwe’s Sandawana mines have been an important producer of emeralds for 40 years. Since the mines came under new ownership in 1993, consistent production has been established and, in addition to the small sizes for which Sandawana is known, greater numbers of polished stones as large as 1.50 ct have been produced. Currently, mining at the most active area, the Zeus mine, is done underground, with the ore processed in a standard washing/ screening trommel plant. Sandawana emeralds can be readily separated from emeralds from other localities. They have high refractive indices and specific gravities. Two amphiboles, actinolite and cummingtonite, are abundant inclusions; albite and apatite are common. Also found are remnants of fluid inclusions. Chemically, Sandawana emeralds typically have a very high chromium content.

corundum, inclusions.5, Madagascar.12, gems

Mineral inclusions in gem-quality sapphires from the new mining area in southern Malagasy (Andranondambo) include calcite, fluorapatite, fluor-phlogopite, fluorite, baddeleyite, zirconolite, potassium-feldspar, anorthite, scapolite (meionite), thorianite (thorianite-uraninite solid solutions), and varieties of spinel, such as green Mg-Al-spinel, spinel-hercynite, and spinel-hercynite-gahnite solid solutions. These minerals are typically found in various rocks in the mining area belonging to the so-called U-Th skarns (various types of calcsilicate marbles). Therefore, these skarns are considered to be the host rocks for sapphire formation. Based on the different generations of primary fluid inclusions in the sapphires, various generations of solid inclusions, and the absence of hibonite inclusions, it is concluded that the Andranondambo sapphires have formed during several different metasomatic events. The later blue sapphire generations have grown from CO₂-rich fluids of complex chemical composition with minor concentrations of fluorine, water, and metals such as aluminium, titanium, iron, chromium, and zinc as well as other elements (Mg, Si, Ca, Na, K, P, Zr, U, Th).

gems, opal, Ethiopia

Opal occurs as nodules in volcanic rocks at a new gem opal locality in the Menz Gishe district of Shewa Province, Ethiopia. The opal field, still in the early stages of exploration, extends over several square kilometers. Opals from Shewa Province have near-colorless to white, yellow, orange, gray, or brown body colors; some show face-up play-of-color, and many have contraluz play-of-color. The gemological properties are consistent with those of other natural opals, and small particles are common inclusions. Preliminary stability tests indicate that much of the material absorbs water, and some crazes when exposed to light and heat.