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, topaz, imperial topaz, Brazil, Minas Gerais, Ouro Preto, properies, mineralogy, gemology

This special issue of the German magazine extraLapis focuses entirely on topaz, following earlier monographs by D. B. Hoover (1992) and Mineralogical Record (1995). The issue opens with a German translation by Maximilian Glas of part of Hoover’s “Historical Perspectives” chapter, covering the etymology of the name topaz, its misapplied names, obsolete trade terms like “Oriental topaz,” and nomenclature issues. A major contribution by Dietmar Schwarz follows, detailing topaz’s gemological and mineralogical properties, crystal structure, geological formation, and key localities (Sanarka in Russia, Katlang in Pakistan, Ouro Preto in Brazil), along with a list of giant cut topaz stones (up to 36,853 ct). Schwarz also discusses causes of topaz color, irradiation and heat treatments, detection methods, and typical inclusions, illustrated by Edward Gübelin’s photomicrographs. The second half of the issue explores historic mining localities: Heiner Vollstädt and Bernd Lahl chronicle the Schneckenstein site in Saxony, once the only significant topaz source in Central Europe and now a preserve; this is followed by a reprint of part of Nikolai I. Koksharov’s classic 1855 paper on Russian topaz from the Urals and Transbaikalia.

corundum, geology.11, Nepal.12, gems

Gem-quality rubies and fancy-color sap-phires have been recovered from dolomite marble lenses located high in the Himalayan mountains of east-central Nepal (Ganesh Himal). First discovered in the early 1980s, these deposits have had only limited and sporadic production because of their isolated locations, high altitudes, and harsh seasonal weather conditions. The pre-cise locations and geology of the two best-known deposits are described. In addition, 27 polished samples and 19 rough crystals and mineral specimens were thoroughly investigated to document their gemological characteristics and crystal morphology. The most distinctive internal features are their color-zoning characteristics, various cloud-like and other patterns, as well as a wide variety of mineral inclusions, some of which have not been seen in rubies from other sources.

gems, gemology, advanced gemology, gem testing, black gem materials, jet, spinel, hematite

Among the most challenging gemstones to identify are those that are black and opaque or nearly so, as virtually any gem material can become opaque due to inclusions, any black opaque material can be cut and polished, and any porous material can be dyed. As a result, identification requires considering a wide range of possibilities, including various minerals, rocks, and man-made substances. While tools like the microscope, refractive index, specific gravity, magnetism, or radioactivity can offer helpful clues, advanced identification methods such as X-ray diffraction and EDXRF spectroscopy are often necessary—though even these may not yield definitive results. Particularly difficult are black opaque members of the pyroxene, amphibole, and spinel mineral groups.

gems, jade, amphibole jade, nephrite, serpentine, archaic jade, China

In this study, serpentine archaic jades from the pre-Han Dynasty were mineralogically analyzed to determine their components. The primary minerals identified were antigorite, chrysotile, and lizardite, which often intergrow intimately, with antigorite being the most common. Secondary minerals such as chlorite, dolomite, magnesite, talc, and brucite were occasionally present in significant amounts. Accessory minerals, including magnetite, chromite, graphite, and limonite, appeared in minor quantities, typically as fine-grained inclusions or veinlets. Quartz and amorphous substances, especially limonite, were frequently found in the weathered or whitened portions of the jade. The color of serpentine archaic jades is closely related to their mineral content. Higher concentrations of the three main serpentine minerals typically result in greener jade. The jade's color is also influenced by the relative abundance of magnesium (Mg), iron (Fe), and chromium (Cr). Higher Fe and/or Cr contents yield greener jade, whereas higher Mg content leads to a more grayish-white appearance. Submajor minerals also affect the color: chlorite enhances greenness, while dolomite, magnesite, brucite, and talc intensify whiteness. Accessory minerals like magnetite, chromite, and graphite contribute to a darker gray hue and reduce transparency. Limonite, a weathering product, imparts a yellowish-brown color. Additionally, magnetite and chromite increase the hardness and specific gravity of the jade, whereas brucite, chlorite, and talc reduce hardness, with talc giving the jade a greasy or pearly luster. The crystal habit, microstructure, and texture of the jade also significantly influence its perceived quality. Long-term weathering after the jade's creation induces three major secondary changes in its mineral composition and texture. Weathering causes the surface to become mottled and pitted, decreasing luster and transparency. It also increases porosity, reducing bulk density and mechanical strength. The most notable change is the whitening of jade," where the surface and sometimes inner parts of the jade turn whitish. This process can be classified into three types: self-pulverization, recrystallization, and substitutional transformation. The "whitening of archaic jade" is not due to calcification or intentional heating to facilitate carving or achieve a white surface."

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

Yet another new locality for natural blue and treatable sapphire surfaces in this description of material from the Cyangugu district of South West Rwanda. Following summary descriptions of the sapphires' geologic setting and apparent origin from alkali basalts, the results of testing 20 samples is reported. Absorption spectra (visible and infrared), quantitative chemistry (microprobe), and microthermometry were performed on two specially prepared samples. Ten prepared specimens were examined by SEMEDS for inclusions, and eight rough samples were also studied by SEM for detailed surface analysis. The appar ently meticulous methodology yielded high-quality detailed information that is a welcome addition to the gemological literature, despite the small sample size. Numerous illustrations also indicate that this material yields fine-quality gems of brilliant blue color. However, no mention is made of its commercial availability or potential supply.

corundum, inclusions.5, Burma.12, gems

Mong Hsu rubies are characterized by a black sapphire core and a ruby rim. This color zoning has been correlated with the presence of the trace elements Ti (violet to black) and Cr (red). Some examples of Mong Hsu rubies show a complex chemical zonation, which results in multiple zones of rubies, violet sapphires and black sapphires within one particular crystal. Rubies of this type have been studied by analyzing the solid and fluid inclusions as well as their chemical zoning. Minerals accompanying the rubies, such as tourmaline (dravite) and tremolite have also been analyzed. The formation conditions of the rubies have been reconstructed following the study of mineral associations and fluid inclusion analyses. Mong Hsu rubies were formed at 2–2.5 kbar and at temperatures between 500° and 550°C. The fluids were found to be water-bearing and multi-volatile CO₂-rich (multivolatile = composed of CO₂, CH₄ and other components, possibly N₂, H₂S and HF). Fluorite inclusions in the rubies indicate that fluids must have contained some HF. During the study of the homogenization temperatures in different populations of fluid inclusions, it was concluded that no major variations in temperature and pressure occurred during the growth of Mong Hsu rubies. However, the chemical zonation patterns found in Mong Hsu rubies and accompanying tourmalines are indicative of variations in the composition of the fluids, e.g. in contents of F, Ti and Cr. A preliminary model is proposed, which explains the cyclic growth of Mong Hsu rubies, with formation of alternating zones of ruby and sapphire by variation in HF concentration in the surrounding fluids. The possible multiple infiltration of fluids from metapelites into dolomites is discussed.

corundum, ruby, synthetic corundum.7, Vietnam.12, gems