corundum, synthetic corundum.7, gems

In the Lab Notes section of the Summer 1987 issue of Gems & Gemology, we reported on unusually strong curved color banding in a synthetic yellow sapphire. More recently, the East Coast laboratory had the opportunity to examine two synthetic yellow sapphires that were unusual in other ways. When no color zoning or inclusions are present in a yellow sapphire, it is standard procedure to use immersion, magnification, and crossed polarizers to look for Plato lines as proof of synthesis. One of the two stones we examined displayed such strong Plato lines that they were actually visible without magnification or immersion (figure 15). In contrast, the second stone showed no discernible Plato lines at all, which is unusual for a flame-fusion synthetic yellow sapphire. This stone was proved to be synthetic when high magnification revealed the presence of several small gas bubbles. An interesting blue synthetic sapphire, also examined in New York, showed two sets of curved color banding that intersected each other at a very shallow angle. This is clearly indicated in figure 16 by the wedge of blue with its point almost dead center in the photo. The wedge marks the boundary between the two sets of banding, which are probably a result of changes in the growth conditions while the boule was being formed.

corundum, inclusions.5, gems

A 1.06-ct blue sapphire was submitted to the New York laboratory for identification. A spall cavity, the fire-skinned surface, and discoid fractures clearly indicated that the stone had been heat treated. However, the dark blue hexagonal color zoning seen through the table (figure 14) is unusual. It appears that a “phantom crystal” within the sapphire was affected by the heat treatment, leaving the edges fuzzy and partially diffused into the stone. When the stone was viewed with a Beck handheld spectroscope, the 450-nm (iron) line was quite strong. Perhaps this was originally a very dark sapphire that had been heated to lighten the color, with the perimeter of the “phantom crystal” remaining dark in spite of the treatment.

feldspar, inclusions, gems

gems, tektite, natural glass

This review introduces tektites as silica-rich natural glasses and examines their physical properties, historical uses, geographical distribution, characteristic shapes, and disputed origins. Tektites are described as containing approximately 68–82% silica and exceptionally little water, with an average water content near 0.005%. Reported hardness is 6–7, specific gravity 2.33–2.55, and refractive index 1.475–1.53; the latter two properties decrease as silica content increases. Colours are principally black or dark green, with brown and grey examples less common. The historical discussion traces their use as tools, amulets, and jewellery and reviews changing explanations ranging from artificial glass and volcanic ejecta to lunar or meteorite-related sources. Chemical differences from manufactured glass and obsidian, low water content, sparse bubbles, and distinctive surface features are presented as important evidence in these debates. Four major strewn fields are surveyed: Australasian, Ivory Coast, Central European, and North American. Their distribution is interpreted as recording discrete events rather than unrelated local formation, and possible associations with the Bosumtwi and Ries impact craters are discussed. Morphological categories comprise microtektites, commonly recovered from ocean-bottom cores; blocky, layered Muong Nong material; splash forms resembling spheres, droplets, dumbbells, and discs; and Australian flanged buttons. Surface pits, grooves, flanges, and spalled areas are considered in relation to aerodynamic shaping, atmospheric ablation, and thermal shock. Internal flow structures, compositional variations, and strain birefringence provide further evidence concerning melting and cooling. Particular attention is given to layered Muong Nong tektites and their comparatively high volatile contents, which may preserve information about precursor materials and proximity to deposition sites. The author reviews evidence favouring an impact-related terrestrial origin, including similarities to continental materials, differences from analysed lunar rocks, limited cosmic-ray exposure, and possible meteoritic metal inclusions. A proposed sequence involves impact melting of terrestrial rock or soil, ejection above the atmosphere, volatile loss and solidification, followed by atmospheric re-entry and surface modification. Competing interpretations and unresolved problems are retained within the discussion rather than presented as settled. A concluding aside treats claims of moldavite’s supposed mystical properties with scepticism and notes their connection with jewellery marketing.

corundum, treatments.6, Sri Lanka.12, gems

corundum, treatments.6, inclusions.5, gems

corundum, inclusions.5, gems

corundum, quality.10, gems

Corundum “type” categories are presented as a practical system for describing the overall color appearance of rubies and blue sapphires, teaching colored stone grading and communicating international market preferences. Developed from a classification introduced by Henry Ho in 1981, the system requires consideration of the combined effects of hue, intensity, tone, fluorescence, pleochroism, inclusions, color zoning and cutting proportions. Types describe appearance rather than geographic origin and are not intended as a ranking of relative value. The author argues that traditional geographic descriptions, such as “Burmese” ruby or “Ceylon” sapphire, can identify recognizable appearances shared by stones from several sources; consequently, origin alone should not determine a stone’s category or justify a quality premium. Definitions distinguish hue position and intensity, assessed through flashes of brilliance, from tone, assessed through the overall lightness or darkness of the stone. Windowing, extinction, pleochroism and zoning receive particular attention because they alter the perceived distribution and strength of color. Five ruby categories are described: Type A, predominantly pure red; Type B, pinkish red with lower color intensity; Type C, slightly orangy red, commonly with inclusions that scatter light and reduce extinction; Type D, blackish red with high tone, strong extinction and weak fluorescence; and Type E, fluorescent purplish pink to red, commonly with silk and comparatively little visible extinction. Four blue sapphire categories are distinguished: Type A, intensely colored, slightly violetish “royal blue”; Type B, “milky blue,” with minute particles or silk that diffuse light; Type C, brighter, lower-toned “cornflower blue,” often with visible zoning; and Type D, dark “inky blue,” commonly associated with iron absorption, color banding and conspicuous greenish-blue pleochroism. The categories are then applied to purchasing preferences reported for Japan, France, Germany, England, Italy, Switzerland, the United States, Australia and the Middle East. These descriptions encompass differences in preferred color, clarity, proportions and finish, while recognizing that purchasing patterns also reflect affordability and resale conditions. Japanese and German buyers are described as favoring comparatively lighter appearances, English and Australian buyers as commonly accepting darker material, and French and Italian buyers as placing greater emphasis on color than on clarity or proportions. Despite these market differences, the author reports broad agreement among connoisseurs on exceptional quality: ruby combining intense fluorescent red with a slight yellow component, and sapphire combining intense, slightly violetish blue with good clarity, moderate tone and a bright appearance. A slight milkiness producing a velvety texture is also prized by some sapphire connoisseurs. Type classification is ultimately recommended as an aid to understanding the interacting causes of color appearance and matching stones to particular markets.

corundum, assembled stones.8, gems

With all the concern to recognise the modern synthetics (Knischka, Chatham, Kashan, Ramaura, Inamori) one would tend to give too quickly a natural origin to a stone presenting at first sight unequivocal inclusions, and thus forget to check for a possible doublet. The two rubies received recently by the writer for identification fall into this category.

gems, minerals, inclusions

Leeder, Thomas and Klemm provide a comprehensive introduction to the study of inclusions in minerals, described as the first German-language book devoted specifically to inclusion research. Following a historical review, they discuss the appearance, classification and genesis of inclusions and establish the physicochemical principles needed for their interpretation. Considerable attention is devoted to fluid inclusions and the H₂O, CO₂, H₂O–NaCl, H₂O–KCl–NaCl, H₂O–CO₂ and CO₂–CH₄ systems. Analytical methods include homogenization measurements, cryometry, pressure determinations and decrepitation, together with determination of solution chemistry, water content, gas composition and isotopic ratios. The authors emphasize the use of inclusions for thermobarometry and reconstruction of mineral-forming conditions, particularly in mineralogy, petrology and economic geology. The book combines theoretical treatment with specimen preparation, instrumentation, measurement procedures and interpretation of results.