inclusions, microscopy, gems

corundum, ruby, sapphire, synthetic corundum.7, synthetic ruby, synthetic sapphire, gems

Chudoba and Gübelin provide a practical guide to distinguishing natural gemstones from synthetic counterparts, combining a short history of artificial gem production with detailed discussion of the principal manufacturing methods and their diagnostic consequences. The Verneuil flame-fusion process, flux growth, and hydrothermal synthesis are described, together with the characteristic structures and inclusions they produce. Particular attention is given to synthetic ruby and sapphire, spinel, emerald, and other important gem materials, with comparisons between natural and laboratory-grown stones based on growth zoning, curved striae, gas bubbles, mineral inclusions, refractive properties, fluorescence, and other microscopic and physical features. The authors emphasize examination with the gemological microscope as one of the most powerful means of separation, supported by numerous photomicrographs illustrating inclusions and growth phenomena in both natural and synthetic gems. Conceived as a concise working manual for gemologists and jewelers, the book brought together the principal identification criteria then available at a time when synthetic gemstones were becoming increasingly sophisticated and commercially important.

beryl, emerald, Austria, gems

Gübelin investigates whether emeralds from Habachtal, Austria, possess characteristics that distinguish them from emeralds of other origins, combining a review of mining history and geology with measurements and detailed microscopic examination. He recounts the deposit’s reported exploitation from antiquity onward and the repeated difficulties encountered by later mining ventures. Its position at approximately 2,100 metres in the Hohe Tauern, hazardous access, prolonged winters, modest production, and short mining licences are presented as obstacles to sustained commercial development. The emerald-bearing zone lies near the contact between central gneiss and surrounding schists. Gübelin interprets its formation through contact metamorphism and the interaction of granitic residual solutions with neighbouring rocks, assigning the beryllium supply to granite and the chromium responsible for green colour to serpentinite. Biotite schist is the principal host, accompanied by tremolite, actinolite, chlorite, and talc-rich rocks; he draws parallels with emerald deposits in the Urals, India, and the Transvaal. Most local beryl is pale or greyish green, while only a small proportion displays fine emerald colour, and abundant inclusions commonly diminish transparency and value. Crystals generally show simple prismatic forms, sometimes flattened according to their orientation within the schist. Measurements on eleven specimens establish average refractive indices of approximately 1.591 and 1.584, birefringence near 0.007, and an average specific gravity of 2.74. Dichroism is generally weak, combining yellowish green and bluish green, and the absorption spectrum is typical of emerald. Chelsea-filter reactions range from pink to red; ultraviolet fluorescence is usually suppressed, attributed to iron, although some specimens fluoresce red and the response under a Stokes fluoroscope is more conspicuous. The principal diagnostic evidence comes from inclusions and internal fractures. Gübelin describes frequent basal cracks, intersecting fracture systems, healed fissures, and liquid-filled cavities or negative crystals, commonly containing two phases and forming dense featherlike patterns. Biotite is the most abundant mineral inclusion, occurring as isolated plates, clustered books, or material extending inward from the matrix, sometimes altered or strongly resorbed. Other identified inclusions include tremolite, tourmaline, apatite, epidote, sphene, and rutile. Emeralds associated with tremolite-bearing rocks are generally clearer and less fractured than those from biotite schist, though often more bluish, and contain green needles, rods, or blades that may cross the entire crystal, terminate internally, form bundles, or bend. Gübelin regards the appearance of these tremolite inclusions as especially characteristic of Habachtal and valuable for recognizing cut stones, while acknowledging resemblance to amphibole inclusions in Ural emeralds. Photomicrographs document the inclusion assemblages, supporting the article’s central connection between geological environment, internal features, and the assessment of geographic origin.

corundum, ruby, sapphire, inclusions.5, phenomena, star, gems

A.S. Tait investigates the cause of asterism in corundum, testing competing explanations involving twinning lamellae, microscopic cavities, and regularly arranged solid inclusions. The study draws on extensive rough and cut material from Montana, North Carolina, Ceylon, Siam, Burma, and Queensland, with basal thin sections permitting examination of inclusion arrangements in transmitted and reflected light without the distortion introduced by curved gemstone surfaces. Microscopic observations show that star-producing material contains rodlike or needlelike inclusions arranged in the basal plane, parallel to the hexagonal prism faces and intersecting in three directions at 60°. When these inclusions are sufficiently abundant and evenly distributed, a cabochon cut with its axis parallel to the crystal’s optic axis displays a six-rayed star. Stars formed in water droplets placed on thin sections provide a direct demonstration linking the optical effect to the included material. Tait also identifies a second set of basal inclusions oriented perpendicular to the first-order prism faces. Although generally too sparse to produce visible rays, these offer an explanation for twelve-rayed stars when sufficiently concentrated alongside the principal set. Determining the inclusions’ mineral identity proves more difficult. At magnifications up to 900×, they appear reddish brown in transmitted light and show apparently square cross-sections, suggesting rutile, but their fineness prevents reliable refractive-index measurement. Microchemical analysis and X-ray powder examination fail because the quantity of included material is too small. Spectrographic analysis therefore compares carefully prepared samples ranging from apparently inclusion-free corundum to heavily included star-producing material. The latter contains titanium at approximately 0.001% and iron at approximately 0.009%, with traces of chromium; the apparently inclusion-free material shows aluminium with traces of chromium and iron. Combining the titanium detection with the observed colour and crystal habit, Tait identifies the needles as rutile, excluding other titanium dioxide minerals on morphological grounds. The article concludes that symmetrically oriented rutile needles account for the asterism of the material examined. Photomicrographs further relate optical appearance to inclusion distribution: predominantly one-directional concentrations produce a cat’s-eye effect, localized arrangements restrict where a star can be obtained, and uniformly distributed needles permit star formation across suitably oriented portions of the crystal.

corundum, quality.10, history.1, gems

A laboratory examination of an unpolished gemstone from one of the St. Hemma jewels (11th century) of the Gurk cathedral, Carinthia, Austria, showed it to be corundum with rutile inclusions, not yellow quartz (false topaz) as labeled. The source of the stone was probably a Far Eastern locality.

jade, jadeite, fei cui, USA, California, gems

Preliminary field investigations conducted in summer 1950 describe jadeite deposits in the Clear Creek area of San Benito County, California, identified as the first occurrences of jadeite in place recognised in the western hemisphere. Eight bodies were located within an oval serpentine mass approximately four by twelve miles across, bordered by Franciscan sandstone and the Upper Cretaceous Panoche formation. Six bodies are lens shaped, completely enclosed by strongly sheared and brecciated serpentine, and no larger than approximately 50 by 200 feet in outcrop. A seventh forms a vein along the contact between serpentine and an enclosed schist block, while an eighth is inferred from float and jadeite-bearing schist near another contact. The lenses commonly exhibit alternating light- and dark-green bands and contain irregular inclusions of natrolite, pectolite, albite, black talcose material and shaly rock. Preliminary examination indicates that much of their jadeite contains a substantial diopside component. Contact assemblages include grossularite, lawsonite, pumpellyite and green amphibole, with locally developed natrolite. The veinlike body can be traced for approximately fifty feet and is generally six to eight inches wide, narrowing locally to two inches. Dark-green margins grade towards a coarser white centre, whose material is exceptionally rich in the jadeite component according to preliminary X-ray work. Branches and smaller veinlets enter the adjacent schist, and analcite commonly lines or fills later fractures and small faults. Complex cross-cutting relationships, apparent brecciation and enclosed patches of contrasting jadeite suggest multiple stages of growth and deformation, although their sequence remains uncertain. The proposed origin involves deformation of serpentine and brittle schist, creation of fractured and open zones, and entry of mineralising fluids, probably residual from the process that produced the serpentine. These fluids are inferred to have deposited an initially more diopsidic pyroxene followed by nearly pure jadeite, with analcite introduced during or after final consolidation. The authors tentatively interpret the vein occurrence, together with contemporary laboratory findings, as evidence that jadeite could form without exceptionally high pressure and at temperatures below approximately 500°C. The studied deposits were considered unlikely to yield high-quality gem material but promising as sources of attractive specimens for collecting and polishing. Additional discoveries were anticipated throughout California’s serpentine belt, with jadeite’s toughness and density offering useful prospecting clues despite its superficial resemblance to serpentine. Verified occurrences elsewhere in California are also briefly listed.

beryl, emerald, inclusions, India, gems

Gübelin supplements earlier descriptions of Indian emeralds with a detailed examination of four faceted specimens, emphasizing the value of microscopic inclusions for distinguishing their origin. He outlines the discovery and development of emerald deposits in the Aravalli Mountains between Ajmer and Udaipur, particularly near Kaliguman and Bhilwara, where wartime prospecting for beryl and mica revealed emerald-bearing rocks. The emeralds occur in biotite schists associated with hornblende, talcose, and chloritic schists and probable pegmatitic intrusions. Gübelin compares this geological setting with deposits in the Urals, South Africa, and especially Austria’s Habach Valley. His four specimens, weighing 0.70–2.42 carats, display colours comparable to fine Colombian emeralds, although their lustre and visible inclusions vary. Because all were cut stones, he could not examine their original crystal forms directly, but inferred a conventional hexagonal prismatic habit from enclosed negative crystals. Refractive indices range from 1.580 to 1.598, with birefringence of 0.0075–0.009, and specific gravities range from 2.739 to 2.781. These comparatively high values resemble those of emeralds from the Urals and Transvaal but do not independently establish geographic origin. The stones show distinct to strong blue-green and yellow-green dichroism. Lighter specimens exhibit the usual emerald absorption spectrum, whereas darker examples show stronger absorption in the red region. All four remain green through the colour filters tested and are inert under ultraviolet radiation; the two lighter stones exhibit faint dull-red luminescence under X-rays. Gübelin concludes that these physical and optical properties must be supplemented by inclusion evidence. The most characteristic features are liquid-filled negative crystals containing movable gas bubbles, aligned parallel to the emerald’s optic axis. At lower magnification they appear rectangular, square, or comma-shaped; stronger magnification and darkfield illumination reveal their crystalline forms and show that the comma-like outlines arise from groups in which one elongated cavity projects beyond shorter companions. These two-phase inclusions are contrasted with the characteristic three-phase inclusions of Colombian emeralds. A second prominent inclusion type consists of biotite tablets and flakes oriented parallel to the basal plane, often with negative crystals arranged perpendicular to them. Minute actinolite rods and calcite crystals may also occur. The biotite association reinforces the comparison with emeralds from the Urals and Habach Valley, while the combined arrangement and morphology of the inclusions provide the principal distinguishing features described. Through numerous photomicrographs, the article presents an early framework for recognizing Indian emeralds by relating their internal features to their geological environment and measurable gemmological properties.

corundum, ruby, sapphire, inclusions.5, Kashmir.11, India.12, gems

Gübelin reviews the diagnostic value of gemstone inclusions and their significance for understanding mineral formation. Improved dark-field binocular microscopy permits examination of inclusions throughout a faceted stone, revealing characteristic differences in their composition, form, orientation, and distribution. Inclusions are classified both by their timing relative to host-crystal growth and as solid, liquid, or gaseous components. Examples from diamond, corundum, beryl, garnet, tourmaline, spinel, moonstone, and fluorite illustrate their use in gemstone identification and, in some cases, determining geographical origin. Particular attention is given to locality-specific inclusion assemblages in rubies and sapphires, three-phase inclusions in emeralds, and characteristic mineral inclusions in garnets. The paper emphasizes that diagnostic value frequently resides in the overall inclusion pattern and its relationship to crystal structure, rather than in an isolated inclusion. Five plates illustrate representative features. Gübelin advocates closer collaboration between gemmology and mineralogy to advance both practical identification and interpretation of crystal growth.

corundum, color.4, gems

Spectrochemical and spectrophotometric investigations of natural and synthetic rubies and sapphires were undertaken to compare their compositions and optical properties and explore possible methods of distinguishing them. For spectrochemical analysis, ruby samples were ground in a boron carbide mortar under acetone to minimise contamination and examined using the Harvey method of semiquantitative analysis. Results for two natural and two synthetic rubies, comprising a light and a dark specimen of each, were expressed as oxide percentages. Alumina constituted 96.5–98.8% of the samples, while chromium oxide ranged from 0.945 to 2.58%, with higher concentrations in the darker specimen of each pair. Silica and minor quantities of copper, cadmium, magnesium, molybdenum and iron were reported in all four specimens. Vanadium was detected in both natural rubies but in neither synthetic ruby. Numerous additional elements, including titanium, were sought but not detected in these four samples. Separate analyses examined the clear core, combined core and outer layers, and outer layers alone of a Linde synthetic star ruby to investigate the material producing asterism. Titanium increased from 0.064% in the core to 0.162% in the outer layers containing dense interlocking crystals. This enrichment supported the suggested identification of the star-producing inclusions as titania, consistent with their microscopic resemblance to rutile needles in natural rubies. Magnesium was also more abundant in the outer layers, whereas chromium concentrations were relatively similar throughout the analysed sections. Spectrophotometric measurements were made with a Hardy recording spectrophotometer on highly polished plates of natural and synthetic ruby and sapphire measuring approximately 2 mm square and 1 mm thick. Visible spectral reflection curves showed broadly similar patterns for natural and synthetic examples of each gem variety, in keeping with their closely comparable chemical and physical properties. Ruby curves exhibited a broad blue-region maximum, a yellow-region minimum and a pronounced rise towards red wavelengths; sapphire curves showed a broad blue-region maximum followed by lower reflection at longer wavelengths. The curves were submitted to Eastman Kodak for investigation of a special filter that might permit rapid differentiation of natural and synthetic corundum. Research remained incomplete, and no wholly satisfactory filter had been obtained.

corundum, beryl, emerald, inclusions, synthetic beryl, emerald, gems