quartz, inclusions, gems

Microscopic and freezing-stage observations are used to investigate conspicuous single-phase and two-phase inclusions in a small amethyst crystal, with particular attention to their contents and unusual crystallographic shapes. Numerous negative crystals occur in two parallel arrays beneath prism surfaces and are sufficiently large to be visible without magnification. Some retain conventional quartz-like forms, whereas many are markedly elongated along the host’s horizontal crystallographic axes. Most appear to contain a single fluid or gaseous phase; a smaller number contain liquid and a bubble. The article introduces heating- and freezing-stage microscopy as methods for examining individual inclusions and obtaining information about fluid composition and mineral formation. Heating can establish homogenisation temperatures, although interpretation requires consideration of pressure, and carbon-dioxide-rich inclusions may rupture before homogenisation. Freezing followed by controlled warming permits observation of melting, sublimation and other phase changes that can constrain salinity, gas identity and fluid density. In the amethyst studied, cooling to −155 °C produced solid bodies that subsequently disappeared at −99 °C and −90 °C. Ice in aqueous inclusions melted at −0.4 °C and −0.6 °C, corresponding to reported sodium chloride concentrations of approximately 0.7 and 1.0 wt%, respectively. These observations establish a dilute aqueous component, but the gaseous phase could not be identified conclusively. It did not liquefy even at the lowest temperature reached, and the solid material formed during cooling could not be analysed directly. Methane is considered the most plausible interpretation because the solid’s morphology resembles that observed in methane-bearing inclusions; nitrogen is discussed as an alternative. If methane was present, its behaviour implies trapping at very low pressure. The author also relates the distorted negative-crystal forms to previous observations that methane can influence quartz growth habits and cavity morphology. Comparisons with methane-bearing quartz from the Swiss Alps support this interpretation without removing the uncertainty over gas composition. The coexistence of different inclusion fillings is taken to suggest that the amethyst did not grow in a purely methane environment. Instead, the author proposes that an aqueous solution and supercritical methane coexisted following a rapid reduction in temperature and pressure. The study combines a methodological explanation with a cautiously interpreted case, demonstrating both the information available from inclusion phase changes and the limitations of identification without direct chemical analysis.

Thailand.12, Nong Bon, Chanthaburi, gems

Moreau gives a first-hand account of a visit to the Nong Bon ruby mine, about 60 km from Chanthaburi and close to the Cambodian frontier. The deposit was worked by removing roughly 20 m of overburden to reach a gem-bearing alluvial layer about 1 m thick; powerful pumps and water jets broke up the material, after which the lighter fraction was washed away and the heavier gem-bearing concentrate collected and hand-sorted. Daily production was irregular, estimated at 1,000–2,000 carats, but only about 300 carats were considered gem material, including perhaps 2–3 carat stones; associated minerals included black opaque spinels, ilmenite, and black corundum. The rough ruby was generally sold not at the mine but at Bor Rai, where miners and dealers negotiated prices in a busy market. Photographs document the mine, hydraulic working, hand sorting, and trading, while a photomicrograph shows faceted inclusions in a Thai ruby.

corundum, USA.12, Montana.12, inclusions.5, gems

corundum, Yugoslavia.12, Macedonia.12, gems

Schiffmann reports gemological observations on rubies from the Prilep area of Yugoslavia (now North Macedonia), describing the characteristics of the material and features useful in its identification. Particular attention is given to the appearance and internal characteristics of the corundum, including inclusions and other properties observed during examination. The study provides an early gemological description of ruby from this Balkan occurrence and documents material from a locality comparatively little represented in the gemological literature.

corundum, ruby, Kenya.12, East Africa, inclusions.5, gems

corundum, inclusions.5, Sri Lanka.12, gems

Natural boehmite single crystals approaching 0.1 mm in size are described from open cavities within a corundum crystal recovered from the gem gravels of the Ratnapura area, Ceylon. The occurrence provides material suitable for relating the morphology, crystallographic orientation, and optical properties of natural boehmite, which generally occurs in crystals too small for single-crystal investigation. Crushing a corundum crystal approximately 6 cm long exposed cavities about 1 mm wide and 3 mm long, lined with slightly yellowish crystals. Identification rests on an X-ray powder pattern matching boehmite, supported by microprobe examination showing aluminium and only minor iron, equivalent to 0.2–0.3 per cent Fe₂O₃. The crystals are platy or stout prismatic, possess perfect cleavage parallel to the plate, and exhibit a lenticular outline in sections parallel to that cleavage. The study resolves inconsistencies in earlier descriptions caused by differing choices of crystallographic axes, adopting the conventional setting in which b is longer than a and a longer than c. Single-crystal precession photographs establish the orientation, while microscopic measurements of interfacial angles permit identification of the forms (010), (001), (101), and (221). Unlike previously described synthetic plates, the Ceylon crystals develop (001) rather than (100), and no cleavage parallel to (100) is observed. Powder diffraction yields unit-cell dimensions of a = 3.695 ± 0.003 Å, b = 12.212 ± 0.006 Å, and c = 2.867 ± 0.003 Å. Systematic extinctions agree with the orthorhombic space group Amam, although the authors retain the possibility that asymmetric hydrogen bonding makes the true structure noncentrosymmetric. Optical orientation is a parallel to γ, b parallel to β, and c parallel to α. Immersion measurements give refractive indices α = 1.648, β = 1.657, and γ = 1.668, each with an uncertainty of ±0.002. Because these uncertainties limit calculation of the optic axial angle, birefringence is measured independently with a Berek compensator, yielding γ–α = 0.020 and γ–β = 0.012. These values indicate a large optic axial angle of approximately 79° and positive optical character, supported by qualitative conoscopic observations. The paper documents a previously unreported mode of occurrence and provides coordinated morphological, X-ray, and optical data for natural boehmite without proposing a formation mechanism for the cavity crystals.

jade, jadeite, fei cui, nephrite, glass, imitations, simulants, gems

Crowningshield reports several glass gemstone imitations examined at GIA’s New York laboratory, emphasizing how convincing colour, internal crystallization, and manufactured structures can complicate identification. A group of green jade substitutes submitted within a few weeks ranged from ordinary translucent glass to partly devitrified material closely resembling fine fei cui. One relatively straightforward imitation was mounted in platinum with diamonds, illustrating that an elaborate setting does not establish the identity of its principal stone. Another fine green example showed an unusual radiating fibrous pattern. Crowningshield relates such crystalline or partly crystalline structures in glass to devitrification and compares them with the fern-leaf appearance of the Japanese imitation sold as meta-jade or Iimori stone. In that ring, the apparent surrounding diamonds and their prongs were simulated by cutting the metal setting itself. A further 31 ct cabochon resembled valuable imperial fei cui but contained irregular crystal-like structures or tufts produced by devitrification. Another imitation was identifiable as glass from prominent gas bubbles, although its refractive index of 1.66 and mottled appearance so closely approached those of fei cui that two trade members wagered on the laboratory’s conclusion. These cases demonstrate that apparently mineral-like inclusions and plausible optical measurements must be considered alongside decisive microscopic evidence. Other glass substitutions included a bracelet purchased as natural emerald crystals, whose components were manufactured hexagonal prisms, mostly green and sometimes incorporating a brown imitation matrix. The report also describes attempts to reproduce cat’s-eye phenomena in glass. An early white product contained visible hollow tubes that scattered light and generated chatoyancy, while later imitations were available in several colours, including honey-like shades and pale blue. Some displayed sharp eyes despite structures too fine to resolve readily at ordinary gemmological magnification. Side views revealed honeycomb patterns, initially attributed tentatively to devitrification. A brownish yellow example resembling chrysoberyl showed a pronounced fibre-optic effect and larger hexagonal areas composed of numerous smaller hexagonal fibres. Its precise manufacturing method remained uncertain, although fused optical fibres were suggested; its refractive index exceeded the instrument’s scale and its specific gravity was 4.19. Photographs document the internal textures and viewing effects. Collectively, the observations show the variety of glass substitutes and the importance of examining structural details from several directions rather than relying on colour, apparent inclusions, jewellery quality, or a single test.

corundum, ruby, sapphire, inclusions.5, gems

corundum, synthetic corundum.7, assembled stones.8, Kashan, gems

Describes the identifying features of Kashan synthetic rubies and a type of ruby doublet combining natural sapphire with synthetic ruby. Rough and faceted Kashan specimens lent by M.J. O’Donoghue were examined against the background of their reputation and relatively high price. These rubies were grown from a flux melt at normal atmospheric pressure and were reportedly no longer in production at the time of writing. The rough crystals differed from natural ruby in habit, forming flattened rhombohedra with partially resorbed surfaces. Their subdued red colour, weak absorption spectrum, and weak fluorescence spectrum indicated a surprisingly low chromium content. Microscopic examination revealed the characteristic flat flux inclusions previously described by Gemological Institute of America investigators as resembling paint splashes, although these occurred on a relatively small scale in the specimens examined. Individual inclusions were elongated, with parallel sides and rounded ends, and occurred in parallel groups. Twisted, lace-like feathers characteristic of many flux-grown stones were also present. Anderson particularly noted the absence of included foreign minerals. The second note extends earlier observations on doublets comprising a natural Australian sapphire crown and a synthetic sapphire base. In the newer construction, a natural sapphire crown was combined with a synthetic ruby base to produce an effective ruby imitation. The blue or greenish natural upper component supplied natural silk, while the red synthetic base dominated the overall colour sufficiently to mask the crown’s blue-green contribution. This combination could therefore present both the colour expected of ruby and inclusions associated with natural corundum. Differential fluorescence provided an important means of detection: under crossed filters or ultraviolet illumination, the natural sapphire crown remained non-fluorescent while the synthetic ruby base glowed bright red. Microscopic examination also revealed an appearance inconsistent with an entirely natural stone. Anderson warned that these composites could deceive many observers unless their construction and identifying characteristics became widely recognised. Together, the notes emphasise the value of examining crystal habit, inclusion morphology, spectral behaviour, and the separate optical responses of a composite stone’s components when distinguishing synthetic and assembled ruby substitutes.

corundum, Finland.12, gems

Corundum-, sapphirine- and kornerupine-bearing rocks from Paaraskalla near Kittilä in Finnish Lapland are described through field observations, petrography, chemical analyses and crystallographic measurements. Investigation followed the discovery of a red-corundum-bearing boulder in 1965 and located numerous additional boulders and several outcrops within a strongly deformed amphibolite–hornblendite formation near Lapland’s major granulite complex. Corundum-bearing material follows a narrow zone broadly parallel to the formation’s foliation. The principal rock-forming minerals are hornblende and plagioclase, with distinctive assemblages containing corundum, gedrite, sapphirine or kornerupine. Corundum is generally pink, white or red, although small deep blue grains were also observed. Crystals commonly measure about one centimetre or less, with occasional larger examples. Panning of weathered rock beneath the moraine yielded abundant corundum, but no gem-quality material was recovered, six polished specimens showed no asterism, and the low concentration and impurity of the crystals made industrial extraction uneconomic. The occurrence is therefore significant chiefly for its mineralogy and metamorphic history. Detailed chemical compositions, optical properties, densities and single-crystal data are presented for hornblende, gedrite, sapphirine and kornerupine. The hornblende is magnesium- and aluminium-rich, while the sapphirine has unusually high alumina and relatively low silica compared with previously analysed examples. Pale bluish green kornerupine, recognised only in the original boulder, represents a new mineral occurrence for Finland and contains relatively little boron. Mineral textures document substantial replacement: corundum and sapphirine are partly converted to chlorite, kornerupine is strongly altered to chlorite and fine mica, and exposed anorthite is replaced by sericite and clinozoisite while shielded inclusions remain fresh. Dravite associated with corundum suggests boron metasomatism. The assemblages are interpreted as products of high-pressure, high-temperature metamorphism near the amphibolite–granulite facies boundary, followed by greenschist-facies retrogression that partly destroyed the diagnostic minerals. Experimental phase-equilibrium results provide comparisons, but the authors caution against transferring them directly to these chemically complex natural rocks. Whole-rock chemistry, accessory chromite and possible replacement of former pyroxene by hornblende favour a mafic–ultramafic igneous precursor, potentially with aluminium- and magnesium-rich layers. Poor exposure and limited evidence nevertheless prevent exclusion of a metasedimentary origin.