gems, spinel, cobalt-blue spinel

gems, spinel, spinel inclusions, root-like inclusions

diamond, inclusions, gems

Gübelin examines mineral inclusions in natural diamonds as evidence for the conditions and processes of diamond formation in the upper mantle. He distinguishes two principal inclusion assemblages: an ultramafic suite characterized by chromium-bearing olivine, chrome-pyrope garnet, enstatite, chrome diopside, and chromite, and an eclogitic suite characterized particularly by pyrope-almandine garnet and omphacitic clinopyroxene, with rarer coesite, corundum (ruby), rutile, kyanite, and ilmenite. Minerals of the two suites had not then been observed together in a single diamond, whereas several inclusions belonging to one suite may coexist, indicating distinct paragenetic environments. Their chemistry, morphology, and relationships to the enclosing diamond are used to argue that diamond and its inclusions crystallized at great depth under upper-mantle pressures and temperatures and that diamond did not originate in kimberlite during its subsequent ascent. Gübelin discusses experimental and mineralogical evidence for crystallization from carbon-bearing ultramafic or eclogitic melts or partially molten material, with H₂O- and CO₂-rich fluids and Fe-Ni-Cu-Co sulphides playing potentially important roles. Sulphide inclusions, particularly pyrrhotite, are considered in relation to possible reduction reactions capable of precipitating carbon as diamond, while trapped gases provide additional evidence concerning the diamond-forming environment. The preservation of nickel-rich pyrrhotite is interpreted as evidence of rapid cooling, and the survival of diamond during transport requires rapid ascent in kimberlitic eruptions. Photomicrographs illustrate characteristic inclusions of olivine, garnet, enstatite, chrome diopside, omphacitic clinopyroxene, chromite, coesite, ruby, rutile, biotite, sulphides, and even diamond within diamond. The latter demonstrate that diamond growth could occur in successive episodes rather than as a single continuous event. Gübelin concludes that mineral inclusions provide direct samples of the otherwise inaccessible environment in which diamonds formed and permit important constraints to be placed on their paragenesis, pressure-temperature conditions, growth history, and subsequent transport to the Earth’s surface.

corundum, treatments.6, gems

corundum, synthetic corundum.7, treatments.6, gems

CIA’s faceting instructor, Bill Kerr, had in his possession a boule of German-produced synthetic corundum. After rounding the larger end and polishing it, he noticed how the three rays crossed each other to form the expected six-ray star and then extended down the sides of the rough boule still attached. This inspired the attempt to cut a cat’s-eye stone oriented 90° to the c-axis of the boule, resulting in the 6.23 ct synthetic cat’s-eye ruby shown on the left in figure 12. On the right of this double-exposure photograph is the same stone viewed on the end and parallel to the c-axis—one-half of the star is now visible. Another section of the same boule (toward the bottom) that was later worked into a stone contained part of the transparent seed crystal with a shallow overgrowth of the asteriated material. This material, when cut, produced a weak star with an open center caused by the non-asteriated seed-crystal section—a very unusual stone indeed (see figure 13).

corundum, treatments.6, diffusion, gems

The first reaction we had to a group of 21 quite clean" blue cabochon natural sapphires received in our New York lab was: "Something is suspicious. Why didn't the cutter facet them?" Sure enough, under immersion the color proved to be surface induced, or as we state on reports: "Natural sapphire with color synthetically enhanced by a surface diffusion process." We are definitely against saying "Natural sapphire, treated color," because it is not the same as, nor as durable as, plain heat treatment. The evidence of surface diffusion of blue natural sapphire cabochons is best seen at the girdle edge (figure 15)."

corundum, inclusions.5, Thailand.12, gems

gems, jade, pyroxene jade, fei cui, guatemala, Mesoamerica, Nicaragua, Mexico, Honduras, Costa Rica, archaeogemology, Mayan design, aesthetics, Olmec, Aztec, Guatemala, archaeology, lapidary, Teotihuacan, obsidian, natural glass

Obsidian production was a major component of Classic period Teotihuacan's economy, with hundreds of workshops producing cores and blades of green obsidian and bifaces of grey obsidian for distribution throughout central Mexico. Although green obsidian was evidently obtained through a state network, grey obsidian procurement was conducted independently by most workshop groups. Most production took place in residential areas beyond the major public structures of the city, but some of these craftsmen also worked periodically within the public precincts where their output would have been at the disposition of state officials. The burden of this levy fell primarily upon those sectors of the industry that were most dependent on the state for support and most involved in external distribution. However, despite this evidence of an important administered element in the Teotihuacan obsidian industry, the workshops apparently maintained their independence from the state. This may have been due to their organization in strong social units that had a long history of independent existence, and to their self-reliance in procurement and distribution during the earlier stages of the industry's development.

gems, jade, pyroxene jade, fei cui, Guatemala, Mesoamerica, Nicaragua, Mexico, Honduras, Costa Rica, archaeogemology, Cuba, Dominican Republic, Olmec, Mayan, Aztec, Mixtec, Toltec, Zapotec

The Dumbarton Oaks Conference on the Olmec, held in late October 1967, was pervaded by the generous and gentle spirit of the “grandfather of Olmec studies,” Matthew Stirling. It was my first conference on a Pre-Columbian theme and my first sight of the great man. He and his wife, Marion Stirling, invited the participants and those associated with Dumbarton Oaks to their home for dinner on Saturday night. It was there that I found myself sitting at the feet of all of the North American archaeologists who had been present at the actual excavation of the great Offering 4 at La Venta—the famous group of thirteen stone figures and six halved celts arranged as a ritual group. They described how it all had to be recorded, photographed and removed in the same day for fear that it might be stolen that night. Robert Heizer told how the next day they were able to pinpoint exactly the balancing offering, which appeared to be wizardry to the laborers. But the most moving part of that conference was Matthew Stirling, the man who had excavated all the important Olmec sites and had unerringly called attention to most of what we still know about that mysterious civilization. His love affair with the Olmec sites began with a blue-jade masquette in the Berlin Museum which he had gone to see in 1920. What attracted Matthew Stirling and all olmecistas from José Melgar through Miguel Covarrubias to the present generation is the remarkable conception and mastery of form and technique in the working of materials—especially the working of hard stones such as jade and serpentine. It might be said that no other lithic culture in world history has been as precise in its choice of stones and as accomplished in the working of them as the Olmec. I would like to discuss Olmec forms and materials from the highlands of Mexico—especially north-central Guerrero.

gems, gem testing, gemology, advanced gemology, advanced testing

In 1948, Raymond Castaing, then a student of A. Guinier, presented to the University of Paris a doctoral thesis entitled “Application of Electron Beams to a Method of Local Chemical and Crystallographic Analysis.” The instrument he described had been produced by modifying an electron microscope, using an electron beam focused to a diameter of less than a micrometer (1 µm = 10⁻⁶ m) to excite X-rays within a microscopic region on the specimen surface. Spectral analysis of these X-rays provided information about the composition of the excited region, while the crystalline structure of the area could also be investigated using the Kossel-line X-ray diffraction technique. Castaing’s electron probe microanalyzer has since become the most important tool for elemental microanalysis, distinguished by its high spatial resolution, nondestructive operation, quantitative potential, wide elemental detection range, and compatibility with a variety of specimen types.