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

gems, peridot, olivine, Pakistan

Following the Fall 1994 Gem News report on a new peridot source in Pakistan’s far western Himalayas, further study of both rough and cut stones from the deposit has been conducted. The orthorhombic peridot crystals may initially appear hexagonal due to the symmetry of “columnar” and “pyramidal” faces, with face types identified using Goldschmidt’s Atlas der Krystallformen. Prominent faces included basal pinacoid (001), columnar (010) and (110), and pyramidal (021) and (111), with striations visible along the c-axis on the (010) and (110) faces, though many crystals, as confirmed by Laura Thompson of Shades of the Earth, also exhibit well-formed, lustrous faces. Two faceted stones—a 5.23 ct rectangular mixed cut and an 18.37 ct cushion modified brilliant—showed gemological properties consistent with known peridot: medium yellowish green color, transparent diaphaneity, R.I. values of α = 1.651, β = 1.669, γ = 1.687 (birefringence 0.036), specific gravity of 3.35, weak pleochroism, inert fluorescence, and distinct absorption bands at ~453, 477, 497 nm, and a weaker one at ~529 nm. These values suggest a composition of ~90% forsterite and 10% fayalite. No inclusions were visible under magnification in the faceted stones, though black rod-like inclusions, identified as ludwigite via X-ray diffraction, were noted in some cabochon-grade rough.

corundum, inclusions.5, Thailand.12, Kanchanaburi, gems

A cobalt-rich spinel has been discovered as a ~200 µm inclusion, alongside a glassy phase, within a gem-quality blue sapphire from Bo Ploi, Thailand, marking the first reported natural occurrence of such a spinel. Its composition closely resembles that of cochromite, a rare cobalt-rich chromite, with electron and proton microprobe analyses revealing A1₂O₃ (48.18–61.27%), CoO (19.7–22.84%), Cr₂O₃ (0–12.28%), FeO (8.64–9.67%), MgO (6.04–6.89%), TiO₂ (0.49–0.73%), and trace elements such as Ni (2251–2532 ppm), Zn (335–371 ppm), and Mn (<177–849 ppm), among others. Notably, transition elements decrease from core to rim, while other elements increase, suggesting a vacancy-creating substitution mechanism (3Co²⁺ = 2Al³⁺ + [□]). The coexisting glassy phase likely formed due to heating by the host basaltic magma. Although further physical investigation is limited by the spinel’s mode of occurrence, this unusual mineral is believed to result from a complex magma mixing process in the lower crust.

corundum, Burma.12, gems

gems, jade, jadeite, fei cui, Guatemala, Mesoamerica, geology

Jadeitites from Guatemala occur as weathered blocks within tectonized serpentinite in a 15-km zone north of the Motagua Fault Zone. Rock types associated with jadeitite include albitites, albite-mica rocks, omphacite/taramitic amphibole-bearing metabasites, chlorite-actinolite schists, talc-carbonate rocks and antigorite schists. In addition to the predominant jadeitic (Jd₉₃–₁₀₀) pyroxene, common phases in jadeitite include micas (paragonite and/or phengite ± rarer phlogopite), omphacite, albite, titanite, zircon, apatite and graphite. Conditions of jadeitite formation are 100–400°C, 5–11 kbar with 0.0 > log₁₀ aSiO₂ ≥ −0.7. Fluid inclusions, coarse textures, vein structures and rhythmic zoning of pyroxene indicate that an aqueous fluid was involved. Jadeitites are either (1) metasomatic modifications of former felsic-to-pelitic inclusions that have undergone silica depletion plus efficient soda exchange and enrichment, or (2) solution precipitates derived from such a source. The close spatial relationship of faults and shear zones, serpentinites and jadeitites suggests that jadeitites formed in a relatively high-P/T setting with substantial flow of sodic fluid in a tectonized zone. Most Guatemalan jadeitites are extensively altered to analcime, albite, taramitic amphibole, (clino)zoisite ± nepheline and preiswerkite. This alteration reflects depressurization/heating to below the jadeite + fluid = analcime reaction at high aNa. With progressive alteration, analcime and nepheline are replaced by albite; the increase in silica content may result from fluid flowing up a tectonized zone reaching saturation with an albite assemblage. Albitite phases, albite, actinolite, zoisite, chlorite, phengite, K-feldspar and quartz record conditions of c. 3–8 kbar at T < 400°C, indicating a clockwise P–T trajectory of the blocks. Barium aluminosilicates—banalsite, celsian, cymrite and hyalophane—are common minor late-stage phases in jadeitites and albite-rich rocks. Barian phengite is common in albite-mica rocks.

corundum, inclusions.5, gems

gems, beryl, emerald, glyptic, engraved gems, emerald bust of Nero, art market, Christie’s 1992

Common gem materials can present challenges in identification when fashioned into intricate carvings or altered by surface conditions. A miniature bust submitted to the East Coast lab—purported to depict the Roman Emperor at age 11–13 (circa 49–51 A.D.)—was examined for authenticity and composition. Due to the detailed carving, refractive index measurements were not possible, but hydrostatic testing gave a specific gravity of 2.70. Spectroscopy revealed weak chromium absorption typical of emerald, and a faint pink reaction under the Chelsea filter further supported this identification. The piece, semitransparent to translucent, displayed numerous fluid inclusions and small, unidentified crystals, one likely being biotite mica. Surface-reaching fissures exhibited yellowish brown iron staining, but no signs of oiling. Based on its properties, the bust was identified as natural emerald. If its reported provenance is genuine, it is probably Egyptian in origin. According to John Sinkankas, emeralds were mined in Egypt during the Graeco-Roman period through the 13th century A.D. Supporting this, Robert C. Kammerling of GIA collected samples from Egyptian mines in 1991 that matched the carving in color, transparency, inclusion patterns, and biotite presence.

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

gems, diamond, North Africa, Algeria, Sahara, Bled-el-Mass Valley

Diamonds and the indicative minerals (pyrope, picroilmenite, and chromite) are found on a vast area of submeridional elongation of 300 km of the Reggan district in the Algerian Sahara. The Bled-el-Mas ancient valley is in its centre, and here Late Pleistocene alluvials and proluvials are diamondiferous. Diamonds are represented by octahedral, dodecahedral and transitional colorless and yellowish crystals with chromite, graphite and other mineral inclusions. Among the pyropes the knorringite-containing varieties of diamond association is found. As an intermediate diamond source the Early Cretaceous conglomerates are regarded, while their primary source has not yet been established.

gems, garnet, almandine, Egypt, Greco-Roman jewelry, ancient jewelry

During a 1991 visit to Egypt, a Gem News editor observed gem-set antiquities in the Egyptian Museum in Cairo, including dark brownish red stones in Greco-Roman jewelry that resembled almandine garnet. A museum chart listed garnet among the materials used by ancient Egyptians, though without specifying species. Later, in Luxor, the editor encountered a necklace of similarly colored beads from a gem dealer, who claimed they were garnets sourced from the south-central Sinai Peninsula, near Gebel Musa (Mt. Sinai), though the exact mine location was lost. John D. Rouse, in Garnet, referenced Pliny’s mention of Egypt as a source of “carbuncles,” likely garnets, possibly originating from a Theban trading station rather than a mine. The necklace, acquired for gemological study, comprised 36 off-round beads, 12.3–13.0 mm in diameter, appearing brown and opaque in reflected light but brownish red and transparent in transmitted light. Magnification revealed dark inclusions and stained fractures. Spot refractive index readings exceeded 1.80, and spectroscopic analysis showed absorption features typical of almandine and spessartine. EDXRF analysis confirmed the presence of iron and manganese, leading to the identification of the beads as almandine–spessartine garnets.