synthetic corundum.7, hydrothermal, gems

Gübelin investigates two recently introduced gem products—rubies represented as hydrothermally grown and natural beryl coated with synthetic emerald—emphasizing the identification difficulties created when synthetic growth surrounds a natural crystal containing convincing natural inclusions. He first reviews the hydrothermal synthesis of corundum developed by Laudise and Ballman at Bell Telephone Laboratories, describing the transport of dissolved nutrient through a temperature gradient in a pressurized vessel and its deposition on suspended seed crystals. Chromium supplies the ruby colour, while growth conditions and the materials used in the apparatus influence the resulting crystals. The ruby specimens personally examined, claimed to originate from Chatham, consisted predominantly of natural ruby cores covered by relatively thin synthetic layers. Their refractive indices, birefringence, specific gravity, dichroism, absorption spectra, and fluorescence provided no reliable separation from natural ruby. The cores contained rutile silk, mineral inclusions, and liquid feathers, while the overgrowth reproduced the original crystals’ external forms. Identification depended on minute features interpreted as gas bubbles within the surface layer, occurring singly or in lines, clouds, and planes, together with the abrupt termination of natural inclusions at the boundary between core and overgrowth. Gübelin notes that ordinary cutting could remove most or all of these thin coatings. The second product, manufactured by Johann Lechleitner and marketed as Emerita, comprised a previously fashioned natural beryl nucleus overgrown with a thin hydrothermal synthetic emerald mantle. The coating continued the crystallographic orientation of the seed, forming a continuous crystal rather than a cemented assembly. Its thickness varied with crystallographic direction and controlled the apparent colour. Standard gemmological properties again overlapped those of natural emerald, and refractometer readings primarily represented the synthetic surface. Microscopy, particularly with immersion and examination from the side, revealed the dark green rim around a pale or colourless core, occasional reflections at the interface, and characteristic structures within the overgrowth. These included parallel or intersecting tension fractures, partly healed fissures containing residual liquid droplets, and particles provisionally interpreted as other mineral phases, such as phenakite or euclase. Some particles obstructed growth and produced tapering cavities containing liquid and a bubble. High magnification and phase contrast helped clarify these delicate features and their relationship to the growth process. Gübelin concludes that natural inclusions within a seed cannot establish the natural origin of the entire stone. Careful examination must instead distinguish the core from its synthetic mantle. He also stresses that altered manufacturing conditions can eliminate many diagnostic inclusions; in exceptionally clean emerald coatings, the contrast between the green outer layer and pale interior may provide the only decisive evidence.

Zimbabwe, Rhodesia, gems

Gübelin examines emeralds from the newly discovered Sandawana deposits in the Belingwe area of present-day Zimbabwe, combining an account of their discovery and geological setting with measurements and microscopic observations intended to establish distinguishing characteristics. Laurence Contat and Cornelius Oosthuizen encountered emerald while prospecting for pegmatite minerals, with an initial discovery in October 1955 followed by a significant deposit found in May 1957. The emeralds occur in association with tremolite schists and pegmatite dikes in the Mweza Range, a setting compared with that of emeralds from Austria’s Habachtal. Geological knowledge remained preliminary, and the workings were still limited, although emerald-bearing material derived from erosion was also recognized. An initial lot of ninety-two emeralds was submitted for investigation. The faceted specimens described are small, weighing 0.15–0.38 carat, but display vivid, comparatively uniform green color with a warm yellow component that remains strong even at these sizes. A reported chemical analysis includes chromium, iron, magnesium, sodium, and lithium in addition to the principal constituents of beryl. Measured optical properties are relatively high for emerald, with representative refractive indices of approximately 1.593 and 1.586 and birefringence near 0.007. Specific gravity ranges from 2.744 to 2.768. Most specimens show the familiar emerald absorption spectrum, a weak red appearance through the Chelsea filter, and distinct bluish-green and yellowish-green dichroic colors. They are inert under the long-wave and short-wave ultraviolet radiation used in the study, with ultraviolet transmission ending near 320 nm. Because these properties overlap those of emeralds from other deposits, Gübelin places particular emphasis on inclusions. The most characteristic assemblage consists of fine tremolite needles approaching actinolite in composition, occurring as short scattered rods and long, slender, sometimes curved fibers. Tiny disk-like fissures along some needles create the appearance of flakes pierced by rods. Compared with Habachtal material, Sandawana emeralds contain finer needles, less densely grouped, with short rods accompanying the longer fibers. Other observed inclusions include brown garnet, limonite coatings and fracture fillings, and rare altered plagioclase, magnetite, and hematite. Dust-like green or brown patches, described as splashes, may form halos around resorbed garnets; an illustration identifies such formations as comprising extremely minute liquid droplets. Additional fissures form interrupted lines and bands parallel to the basal plane, differing in appearance from the more extensive layers of disk-like fissures described in Russian emeralds. Gübelin concludes that the combination of physical properties, tremolite morphology, internal irregularities, and vivid color provides a practical basis for recognizing Sandawana emeralds, while emphasizing the importance of direct observational experience.

inclusions, microscopy, gems

Gübelin explains the principles of phase-contrast microscopy and describes equipment adapted to examining inclusions and subtle internal structures in gemstones. Developed from F. Zernike’s work, the method converts differences in the phase of transmitted light, arising from variations in thickness or refractive index, into visible differences in brightness. It thereby reveals transparent features that produce insufficient contrast under ordinary bright-field illumination. Closing the condenser aperture or defocusing can improve conventional contrast but reduces resolving power, while dark-field illumination chiefly emphasizes sharp boundaries and may inadequately display gradual transitions or details within inclusions. Phase contrast employs an annular illuminating diaphragm and a corresponding phase plate in the objective to modify the relationship between direct and diffracted light. Positive phase contrast renders optically denser features darker than their surroundings, whereas negative phase contrast reverses this relationship. Conventional equipment initially presented difficulties because its high-power objectives had insufficient working distance and depth of focus for inclusions situated within relatively thick gemstones. With assistance from H. Piller of Zeiss-Winkel, Gübelin obtained a specially designed arrangement incorporating suitable illumination components, annular diaphragms, and objectives of lower magnification with longer working distances. An auxiliary microscope permits alignment of the illuminating ring and the objective’s phase ring. The arrangement allows convenient changes between phase contrast and conventional illumination, facilitating comparison and interpretation of the same features. Its principal value lies in increasing the visibility of inclusions and compositional irregularities, rather than providing quantitative refractive-index measurements. Comparative examples include muscovite flakes in yellow beryl from Madagascar, structures in a Colombian emerald attributed to incomplete calcite exsolution, alteration to pinite in Sri Lankan iolite, and partly healed fissures in sapphire, fluorite, and beryl. Phase contrast makes the boundaries and internal details of muscovite conspicuous despite its small refractive-index difference from beryl. It also distinguishes residual liquid-filled portions of healed fractures from the surrounding healed material and can reveal gas bubbles within those residual cavities. Structural irregularities and zoning may become visible as contrasting light and dark areas where exsolution, precipitation, alteration, or healing has produced slight local differences in refractive index. Gübelin emphasizes that the method is most useful when optical differences are small and the relevant structures sufficiently thin. It offers little advantage for many ordinary inclusions whose refractive indices differ greatly from those of their hosts, and it does not reveal strain or deformation caused by stress. The article presents phase contrast as a valuable addition to gemological microscopy for investigating internal heterogeneity and interpreting processes recorded within gemstones.

corundum, history.1, gems, jade, Fischer-Other, early lapidaries

According to Caley and Richards (ibid., 4), the treatise is a set of notes, possibly taken down by a student listening to lectures delivered by Theophrastus, but “safely assumed to represent the actual views of Theophrastus, and he may reasonably be considered the author.” On the other hand, Eichholz (ibid.) is firmly of the view that the treatise is Theophrastus himself, and that it is not a fragment, as has been suggested numerous times, but a complete, though short, work. Theophrastus’ treatise, commonly called the Peri Lithon, or “Concerning Stones,” commences with speculations as to the origins of stones and discusses their distinctive properties as well as the means by which differences among them may be elucidated. This is followed by a rather rambling description of numerous gemstones and earths, excepting metallic minerals and metals, which are stated to have been described elsewhere in a treatise now lost. All of the matter is presented tersely and is almost entirely free of superstitious lore, except for the emerald, for which it claims that it refreshes the eyesight upon gazing and that an emerald placed in water colors it green. F. D. Adams (Birth and Development of Geological Sciences, 1938, 19) notes that “it differs entirely in its style and content from any other work upon this subject written in classical times—in fact, no other work at all resembling it appeared until later on in the Middle Ages. It remained for 1800 years a most valuable and authoritative work on minerals, referred to and quoted by all, and shows that even at this early time there existed, probably among miners, quarrymen and other men engaged in the technical trades and industries, a body of knowledge of mineralogy, especially on its practical side, which had been drawn upon by Theophrastus in the compilation of his work.” Adams further notes that Pliny must have had a more complete version of the treatise because of mentions of stones that are not now in the Peri Lithon, but this is refuted by Eichholz as noted above. H. Tertsch (Das Geheimnis der Kristallwelt, 1947, 31–4) also provides illuminating remarks on the treatise, suggesting that it reflects the views of Aristotle, his mentor. The gemstone content of Peri Lithon is not large but important for those ornamental and precious materials that it does name—namely, marbles and calcite onyx, lapis lazuli, obsidian, and other massives, and also emerald, sard, garnet, adamas (probably corundum), amber, rock crystal, and others—all of which are carefully examined by Caley & Richards and by Eichholz, who make identifications where possible. Both authorities offer much ancillary material which makes rewarding reading as well as providing lists of printed editions of Peri Lithon from which I have freely drawn much of the information below on not personally examined.

gems, pearls, precious stones, jewelry

Hatem presents a popular but substantial survey of pearls and precious stones, combining gemological information with history, geography, mining, trade, and famous gems. She discusses the principal gemstones, including diamond, ruby, sapphire, emerald, and pearl, describing their formation, physical characteristics, important producing regions, methods of recovery and cutting, and their place in commerce and jewelry. Historical accounts and anecdotes trace the fascination with precious stones from antiquity to the modern period, including celebrated jewels, royal treasures, discoveries, and the development of major gem-producing districts. Synthetic gemstones and attempts to manufacture diamond are also treated, including Moissan’s experiments. Pearls receive extensive attention, with discussion of natural pearl fisheries, cultured pearls, mollusks, formation, grading, and important producing regions. Written for the general reader rather than as a technical gemological manual, the volume brings together scientific explanations and historical material and is extensively illustrated, with 78 figures and a map in the text and 103 illustrations on plates.

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, history.1, Sri Lanka.12, pearls, gems, jade, Fischer-Other

A facsimile reproduction of Arthur Golding’s 1587 English translation of the Collectanea Rerum Memorabilium (Collection of Memorable Things) by the third-century Roman writer Caius Julius Solinus, also known as Polyhistor, with an introduction by George Kish. Drawing extensively on Pliny the Elder’s Natural History and other classical authorities, Solinus presents an encyclopedic geographical survey of the ancient world, describing its countries, peoples, customs, animals, plants, minerals, and natural wonders. Particularly valuable for the history of gemology are numerous accounts of precious stones, their geographical sources, physical characteristics, supposed medicinal virtues, and supernatural properties. Among the materials discussed are emerald, sapphire (under the name cyanus), crystal, agate, jasper, onyx, sardonyx, coral, amber, and numerous obscure stones known principally through classical lapidary traditions. Solinus describes the celebrated emeralds of Scythia, their exceptional green colour and beneficial effect upon eyesight, as well as the difficulties of obtaining them from territories supposedly guarded by griffins. His descriptions of agates include the famous stone belonging to King Pyrrhus, in which the nine Muses and Apollo were believed to appear naturally without carving. Other passages discuss the legendary lyncurium, supposedly formed from the urine of the lynx, and various stones credited with extraordinary powers. The work also preserves ancient traditions concerning the mineral wealth of India, Ethiopia, Egypt, Arabia, and other regions associated with the early gemstone trade. Although much of the material is fabulous or derived uncritically from earlier writers, Solinus exerted considerable influence upon medieval geographical literature, encyclopedias, and lapidaries. Kish’s introduction places the author within the intellectual and political circumstances of the declining Roman Empire and traces the extraordinary popularity of his writings through the Middle Ages and Renaissance. The volume is an important source for understanding the transmission of classical mineralogical knowledge, gemstone lore, and geographical misconceptions into early modern Europe.

corundum, synthetic corundum.7, stars, gems

THE aluminum oxide known as corundum has several varieties that have been used as gem materials since ancient times. These include the red variety called ruby, the blue variety sapphire, and the asteriated types of ruby and sapphire. Asteriated sapphire was known to the ancients under the name of astrios, and descriptions of this gem were given by Pliny and other early writers. The relation of these gem materials to each other as varieties of a single species, corundum, was first clearly established by Haüy in 1805 and J. M. Güthe1 in 1809. Natural gem corundum is obtained principally in Ceylon, Siam, Burma, India, and Australia. Star sapphires and rubies always have been highly prized and in recent years have become particularly popular in the United States. Natural star rubies of large size and fine quality are extremely rare and rank with large and fine emeralds and diamonds as the most valuable of gem stones. Interest thus attaches to the recent success in commercially synthesizing star sapphires and rubies of a size and perfection unmatched by nature.

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.

beryl, emerald, Colombia, Chivor-Somondoco, gems

Nice description of the emerald mines in Colombia, particularly at Chivor.