corundum, synthetic corundum.7, gems

This well-illustrated article examines (by SEM-EDX analysis) the inclusions in Russian hydrothermal ruby and sapphire manufactured by Tairus. Coatings on rough samples were found to be either boehmite or an unknown noncrystalline gel-like material. The following inclusions were observed in both rough and cut samples: solid inclusions of corroded copper (probably remnants of wire), isometric crystals and hexagonal platelets of copper, clouds and linear series of pinpoints of whitish reflective particles, tiny needles, and whitish particles (possibly alkali or calcium carbonates from the mineralizers). In pinkish red to red samples, the authors found fluid inclusion “fingerprints” or feathers, among which occur large three-phase (liquid-gas-mineral) inclusions associated with large tube-like negative crystals. The daughter minerals in the inclusions were identified (by crossed polarizers and heating/freezing [H/F] experiments) as carbonates. H/F experiments also revealed that the liquid phase is predominantly H₂O. By contrast, three-phase inclusions in natural rubies are often CO₂-rich with daughter minerals of diaspore, graphite, or mica. Of perhaps greatest interest to the practicing gemologist, the authors describe a simple gemological H/F test to distinguish between natural and synthetic hydrothermal rubies on the basis of three-phase inclusions. The copper inclusions also are indicative of synthetic origin.

corundum, color.4, quality.10, Sri Lanka.12, padparadscha, gems

Notari investigates the physical and chemical characteristics of padparadscha sapphire to clarify inconsistent definitions of its colour and identity. Drawing on historical research, observations in Sri Lanka, professional opinions, and laboratory analyses, he defines its characteristic colour as orangey pink of pastel to medium saturation. Four appearances are distinguished: “original,” “antique,” “induced,” and “deceptive,” with only the first two accepted as genuine padparadscha. The others owe their appearance to treatment or yellowish iron oxides in fractures. Chemical and spectroscopic investigations examine the contributions of chromium, iron, and incompletely understood colour centres, together with blue growth zoning and sunlight-induced colour changes. Raman analysis identifies native sulphur within a multiphase inclusion containing diaspore, graphite, rutile, and carbon dioxide. The article discusses heating and irradiation treatments, their detection, and diagnostic features of melt-grown, flux-grown, and hydrothermal synthetics. Notari concludes that padparadscha is a colour variety rather than a geographical designation, while noting that only Sri Lankan production then regularly supplied stones matching the complete suite of characteristics described.

gems, tourmaline, Kenya

Tourmaline from a recent find in Kenya, referred to as “savannah” or “golden” tourmaline, has attracted attention for its distinctive coloration. Gemologist Yianni Melas submitted two rough samples for examination: a 4.50 ct transparent brownish orangy yellow piece with green areas, and a 3.06 ct crystal section showing a brownish orangy yellow core with a green rim. The broken crystal section displayed only prism faces, lacking striations parallel to the c-axis. Gemological properties recorded include pleochroism from brownish orange to yellow in the yellow regions and dark green to light yellowish green in the green areas, uniaxial optic character, refractive index of 1.630–1.650 with a birefringence of 0.020 (on the green rim), and specific gravity of 3.05. Under long-wave UV, the stones were inert, but showed moderately chalky yellow fluorescence under short-wave UV, stronger in the green rim. Spectroscopic analysis showed general absorption to ~480 nm, with additional bands between 590–630 nm and lines at 650 and 680 nm in the green rim. Inclusions observed included anhedral crystals, needles or tubes, two-phase inclusions, and hollow tubes. EDXRF spectroscopy revealed Mg, Al, Si, K, Ca, Ti, Fe, Zn, Ga, and Sr in both samples, with one showing a small Mn peak. Chromium was detected in the green rim, consistent with the observed color-filter and absorption data, indicating Cr as the chromophore for the green. Reported sources for this material include the Kasigau Mine in Tsavo West National Park, the Mangari area in the same park, and M’gama Ridge south of the Taita Hills in southeastern Kenya.

gems, beryl, emerald, Pakistan, Swat, emerald chemistry

Emerald deposits in Swat, northwestern Pakistan, occurring in talc-magnesite and quartz-magnesite assemblages, have been investigated through stable isotope studies. Isotopic analyses were performed on a total of seven emeralds, associated quartz (seven samples), fuchsite (three samples) and tourmaline (two samples) from the Mingora emerald mines. The oxygen isotopic composition (δ 18O SMOW) of emeralds shows a strong enrichment in18O and is remarkably uniform at + 15.6 ± 0.4‰ (1σ,n = 7). Each of the two components of water in emerald (channel and inclusion) has a different range of hydrogen isotopic composition: the channel waters being distinctly isotopically heavier (δD = −51 to −32‰ SMOW) than the other inclusion waters (δD = −96 to −70‰ SMOW). Similarly the oxygen isotopic compositions of tourmaline and fuchsite are relatively constant (δ 18O = + 13 to + 14‰ SMOW) and show enrichment in18O. Theδ 18O values of quartz, ranging from + 15.1 to + 19.1‰ SMOW, are also high (+ 16.9 ± 1.4‰ 1σ, n = 7). The meanδD of channel waters measured from emerald (−42 ± 6.6‰ SMOW) and that of fluid calculated from hydrous mineralsδDcalculated (−47 ± 7.1‰ SMOW) are consistent with both metamorphic and magmatic origin. However, the close similarity between the measuredδD values of the hydroxyl hydrogen in fuchsite (−74 to −6‰ SMOW) and tourmaline (−84 and −69‰ SMOW) with pegmatitic muscovite and tourmaline suggests that the mineralization was probably caused by modified (18O-enriched) hydrothermal solutions derived from an S-type granitic magma. The variation in the carbon and oxygen isotopic composition of magnesite, locally associated with emerald mineralization, is also very restricted (δ 13 ∼ −3.2 ± 0.7%, PDB;δ 18O ∼ + 17.9 ± 1.27‰ SMOW). On the basis of the isotopic composition of fluid (δ 13C ≈ −1.8 ± 0.7‰ PDB;δ 18O ≈ + 13.6 ± 1.2‰ SMOW calculated for the 250-550°C temperature), it is proposed that the Swat magnesites formed due to the carbonation of previously serpentinized ultramafic rocks by a CO2-bearing fluid of metamorphic origin.

corundum, geology.11, Australia.12, gems

Corundum occurs widely in eastern Australia, especially in New England which hosts the world's largest producing sapphire mine at Kings Plains. The alluvial deposits of New England show strong local tribal variations in the nature of the sapphire, suggesting that they are derived by reworking of a source rock that is locally rich in corundum. Studies of corundum textures and mineral inclusions suggest a variety of origins, but the commercial sapphire deposits occur in regions where Tertiary alkali basalts are prominent and governed by episodic revival of volcanism over a prolonged period of time. Corundum is brought to the surface in basaltic eruptions as xenocrysts, but it is found only rarely in basalt lava. Throughout the New South Wales Tertiary volcanic province there are basalt capped remnants of thin units of clay-altered basaltic volcaniclastic rocks. These rocks display many features found in basaltic diatremes and are interpreted as thin distal aprons produced by base surge flows during an early violent stage of eruption. They carry elevated grades of sapphire, erode quickly and would have been laterally extensive. They are considered to be the major source of alluvial sapphire in New England and in similar provinces elsewhere. Their whole-rock geochemistry, using residual element ratios, indicates a regional link between the more fractionated volcaniclastic compositions and significant sapphire deposits.

corundum, geology.11, gems

Zircons enclosed in corundum megacrysts from basalts exhibit a short prismatic habit dominated by the {110} prism over {100}, consistent with crystallization from alkaline–peralkaline environments. Occasional zircon decomposition textures hint at the presence of a primary SiO₂ phase in the source paragenesis. These zircons are geochemically distinctive, with elevated contents of Y (up to 1.2%), U (UO₂ to 1.7 wt%), Th (ThO₂ exceeding 2 wt%), Hf (up to 3.4 wt%), and total REEs (up to 5000 ppm), and they show extreme REE fractionation, particularly enriched in HREEs (30–100× chondritic Sm and 5000–20000× chondritic Lu). Evaluated zircon/silicate melt partition coefficients indicate that the parental melts of these zircon inclusions had concave-down REE patterns peaking near 100× chondritic Sm, reflective of highly evolved melts formed after extensive low-Ca feldspar fractionation. These findings support that corundum megacrysts crystallized from highly evolved, alkaline melts under very reducing conditions, and were not high-pressure phenocrysts crystallized directly from mafic host magmas.

gems, glyptic, engraved gems, agate, cameo, intaglio, Rome, Roman gems, history of gemology

A Roman gold signet-ring (Pl. XXb) was discovered in 1974 in a garden at Brackley, Northamptonshire, and was later submitted by the finder for examination at the Buckinghamshire County Museum, Aylesbury. The find-spot, located in Watery Lane, lies just north of a building that has yielded pot-sherds, tile, brick, slate, tesserae, and flint—suggesting a possible villa, though Roman activity appears widespread in the area, indicating the building may have been part of a larger settlement. The ring is made of high-purity ancient gold and features a hoop with a rounded section that expands from 3 mm at its narrowest point to 6.5 mm at the bezel; its external width is 18 mm (internal width 12 mm) and its height, including the stone, is 16 mm (internal height 11 mm). Set into the bezel is a green translucent stone with dark inclusions, initially believed to be a chrome-rich chalcedony (plasma), as is common in Britain, but identified through X-ray fluorescence analysis by the British Museum Research Laboratory as an emerald. Known in antiquity as smaragdus, emeralds were highly prized though rarely engraved due to their hardness—making this the first known emerald intaglio from Roman Britain. The gem, which has a convex profile and measures 8 mm by 5 mm, is engraved with the image of the corn-goddess Ceres (Greek, Demeter), sometimes conflated with the Roman Fides Publica on coins. She is depicted standing frontally, dressed in a belted chiton, holding two heads of corn in her left hand and a dish of fruit in her right—an image that would appear in reverse in impression (Pl. XXc).

corundum, inclusions.5, treatments.6, synthetic corundum.7, Tanzania.12, gems

gems, spinel, spinel inclusions, Tanzania

corundum, synthetic corundum.7, Czochralski, gems

This past summer, a manufacturer of synthetic corundum shared the Czochralski-pulled synthetic ruby shown in figure 20 with staff mem-bers of the East Coast lab. The modi-fied triangular mixed cut weighed 115.56 ct and was completely trans-parent, with a strong red color that had no purple or brown components. Although natural ruby may be found in large sizes, it usually contains so many inclusions that the diaphaneity is translucent rather than transparent, One area of the pavilion of this large synthetic showed fine curved striae, but otherwise it was free of inclusions or variations in color. Because of the large size and trans-parency, a slight doubling of the pavilion facet junctions-a rare sight in corundum, which has low birefrin-gence-was visible through the table. The manufacturer of this synthetic ruby reports that he is using a new process in which growth is controlled on a scale of several inches, with the goal of producing material that is entirely free of inclusions or disrup-tions of the crystal structure (such as twinning or graining) up to sizes as large as 500+ ct.