corundum, inclusions.5, gems

corundum, quality.10, synthetic corundum.7, gems

gems, beryl, emerald, Brazil, Santa Terezinha

Emeralds from Santa Terezinha de Goiás, a major global producer, contain a diverse range of primary inclusions identified through X-ray diffraction, energy dispersive analysis, and electron microprobe techniques. These inclusions include chromite, dolomite, magnesite, talc, phlogopite, pyrite, quartz, beryl, barite, patronite, ferropargasite, anthophyllite-cummingtonite, epidote, melanterite, and dumortierite, which reflect the metamorphic nature of the host rocks. Chromite, the most characteristic inclusion, appears as isolated or clustered idiomorphic crystals that exhibit a distinctive red-brown hue under magnification. Carbonates like dolomite and magnesite occur as rhombohedral crystals, while talc is present as dispersed flakes. Other notable inclusions include yellow cubic pyrite, phlogopite (less common than in emeralds from Itabira and Socotó), idiomorphic beryl, and quartz filling fractures. Barite and patronite are rare, while epidote, cummingtonite-anthophyllite, dumortierite, and melanterite were detected only in mixtures via X-ray methods. Fluid inclusions, visible only at high magnifications, contain gas, liquid, and solid phases. The observed inclusions align with the paragenesis of the metamorphic country rocks, and chemical analysis indicates significant variations in the crystallization conditions of these emeralds.

gems, beryl, emerald, Colombia, inclusions

Emeralds from the district Somondoco (Colombia) were analyzed by electron microprobe, colorimetry (Be), emission spectrography (Li), and thermogravimetry and infrared absorption (H₂O). They typically show the following atomic substitutions (per 18 oxygens): 0.13–0.21 Al in the SiO₄ tetrahedra, 0.14–0.40 for the sum of Cr, V, Fe, Mg in the AlO₆ octahedra, and 0.02–0.06 Li plus 0.00–0.170 in the BeO₄ tetrahedra; the “channel” alkalis, mostly Na, total 0.13–0.29. Fluid inclusions in these emeralds contain liquid H₂O, liquid CO₂, daughter crystals of halite and calcite (?), and liquid and gaseous hydrocarbons. The behaviour of the fluid inclusions on freezing and heating and their frequent decrepitation, stretching, and leaking are described. Temperatures of homogenization (Th) values are ≥ 470 °C, but only those close to the lower limit seem unaffected by inclusion stretching, etc. A roughly estimated pressure (≤ 1 kbar) would require a pressure correction of about 100 °C to the Th values. The total salt concentration in fluid inclusions is close to 40 wt.%, and the dissolved salt composition inferred from the electron microprobe analyses of the precipitates from decrepitated inclusions is probably: Cl, Na > Ca, Si, Al, (C) > Fe, K, Mg > Mn, Ti. The behaviour of other possible components in the mineral-forming process is also discussed. It is calculated that beryllium fixed in emeralds makes up only 0.2 to 1 wt.% of the total Be in the emerald-bearing rocks and veins. The proposed model of the origin of emeralds involves only components mobilized from the country rocks, without recourse to a deep-seated or magmatic source.

corundum, inclusions.5, synthetic corundum.7, gems

kyanite, gems

corundum, Nepal.12, gems

Describes the examination of an unusual 1.288 ct faceted ruby from Nepal whose identification as natural or synthetic was initially difficult because several internal features resembled those of flux-grown synthetic ruby. Microscopy revealed dark inclusion veils, negative crystals, unusual growth structures and inclusions showing interference colors; its absorption spectrum and strong red to orange-red fluorescence also appeared compatible with synthetic material. Microprobe analysis, however, identified exposed dark inclusions as phlogopite rather than flux residue. Further examination revealed very thin fluid films, some two-phase, together with twinned mineral crystals and twin lamellae in the host ruby, establishing its natural origin. The paper illustrates the danger of relying upon individual inclusion features when distinguishing natural from synthetic ruby and demonstrates the value of combining microscopy with chemical and spectroscopic examination.

gems, spinel, Sri Lanka, sillimanite inclusions

A bluish-violet spinel from Sri Lanka with oriented inclusions was examined using microscopy, X-ray single-crystal diffraction, and electron microprobe analysis. The specimen displays four sets of lath-like, double-refracting crystal inclusions aligned parallel to the threefold axes of the host spinel. The inclusions are identified as one of the Al₂SiO₅ polymorphs, most likely sillimanite. The study also includes a general literature review on oriented inclusions and asterism in spinel.

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.