corundum, cat’s eye star sapphire, Burma.12, phenomena, inclusions.5, gems

Schmetzer and Kiefert examine an unusual chatoyant sapphire from Burma in order to determine the cause of its cat’s-eye effect. Microscopic examination shows numerous fine, elongated inclusions arranged in a strongly preferred crystallographic orientation. The authors study their orientation in relation to the corundum crystal structure and show that reflection and scattering from these oriented inclusions produce the sharply defined eye. The stone’s internal growth structures, inclusions, optical properties and absorption characteristics are also described and compared with those of other sapphires. The study illustrates how crystallographically oriented inclusions can produce chatoyancy in corundum and documents a relatively uncommon phenomenal variety of Burmese sapphire.

corundum, ruby, sapphire, Nepal.12, inclusions.5, gems

Kiefert and Schmetzer describe pink and violet sapphires from a newly recognized occurrence in Nepal. Associated minerals include calcite, phlogopite, margarite and rutile. The pink color is attributed principally to chromium, whereas the violet coloration results from a combination of Cr, Fe and Ti. The corundums commonly contain twin lamellae and dense sets of straight, parallel growth planes that separate zones differing in color and concentrations of fine mineral particles. Particularly characteristic are small, doubly refractive mica platelets, identified as margarite, some occurring as the solid component of three-phase inclusions. Numerous healed and unhealed fractures are also present and substantially reduce the gem quality of much of the material.

corundum, ruby, Nepal.12, inclusions.5, geology.11, gems

Ruby reportedly originating from a new occurrence in Nepal is characterized through examination of two cut stones, a large prismatic crystal in a carbonate-rich matrix, and corundum-bearing mica schist. The precise locality was undisclosed, and a possible connection with previously reported material from Taplejung remained unconfirmed. Initial examination of a 7.90 ct cabochon and a 2.17 ct faceted stone raised identification difficulties because curved, twisted feather-like inclusions resembled features observed in several synthetic rubies. Subsequent study of rough specimens demonstrated comparable fluid-bearing fractures and undulating films in natural corundum. Straight blue sapphire bands occur within the rubies, and a section through the prismatic crystal reveals a strongly coloured ruby core surrounded by paler red zones and sharply defined blue bands with growth lines meeting at 120°. Accurate refractive indices measured on the polished section are 1.760–1.768. Specific gravities of the two cut stones are 3.99 and 3.98, while the inclusion-rich crystal section has a lower value of 3.79, attributed to abundant margarite. Ultraviolet–visible spectroscopy of differently coloured zones yields predominantly ruby-type absorption patterns, with additional absorption associated with the blue regions. An ultraviolet absorption minimum at 361 nm supports natural origin under the comparative method employed. Electron-microprobe analyses detect up to 0.32 wt% Cr₂O₃ and 0.30 wt% TiO₂, although titanium content does not correlate consistently with blue colour intensity; iron remains below the analytical detection limit of approximately 0.2 wt% oxide. Identified inclusions and associated minerals include margarite, muscovite, zoisite, epidote, rutile and apatite, with some apparently black apatite owing its colour to graphite. The prismatic crystal’s matrix consists chiefly of calcite, dolomite and margarite, whereas the schist is dominated by phlogopite with apatite, graphite and corundum. Ruby fluoresces red to pink, more strongly under long-wave ultraviolet radiation, while margarite displays conspicuous greenish-white fluorescence and sapphire zones fluoresce less strongly. Margarite mantling and intersecting corundum is tentatively interpreted as partial replacement following changes in metamorphic conditions. The mineral assemblage resembles carbonate-associated ruby occurrences at Mogok and in Pakistan’s Hunza Valley. The study emphasizes both these geological affinities and the identification risks posed by natural inclusions that resemble those of synthetic ruby.

corundum, sapphire, inclusions.5, Malawi.12, gems

Grubessi and Marconi describe a complex three-phase inclusion in a yellow corundum crystal from Malawi, interpreting its form as the result of necking-down after initial entrapment. In this process, an originally larger inclusion becomes constricted and subdivided into smaller cavities, some of which may appear isolated within the host crystal. A schematic sequence illustrates the proposed development as temperature decreases, while photomicrographs taken under different illumination conditions document the resulting internal structures. The principal inclusion comprises two large cavities joined by a narrow hollow channel and surrounded by smaller cavities forming a fingerprint pattern. The authors regard the connected cavities as a single inclusion containing solid, liquid, and gaseous phases. The solid component consists of fine, radiating and intersecting birefringent needles, reddish golden in reflected light and tentatively identified as rutile. The liquid is considered possibly carbon dioxide. A mobile vapour bubble changes position within the cavity system and disappears on heating to 31–32 °C, an observation described as homogenization. Photographs under direct illumination and polarized light show the bubble’s movement towards the constricted connection and its disappearance within the channel between the cavities. Small triangular or tetrahedral markings within the larger cavity are interpreted as possible crystallization nuclei. The host crystal is characterized by a vitreous lustre, hardness of 9, density of 4.00 g/cm³, refractive indices of 1.769 and 1.759, and birefringence of 0.010. No pleochroism was observed. A hand spectroscope revealed an absorption band at 690–700 nm, and ultraviolet examination produced weak yellow luminescence under long-wave radiation but no response under short-wave radiation. Other internal features include fractures and twins, and a table presents an energy-dispersive X-ray chemical analysis. The report emphasizes the morphology and behaviour of the inclusion rather than establishing definitive compositions for its constituent phases. Comparisons with necked-down inclusions previously observed in North Carolina amethyst, Montana sapphire, and other minerals place the Malawi example within a broader class of post-entrapment modifications.

corundum, inclusions.6, synthetic corundum.7, Burma.12, gems, Kashan, Ramaura, Linde, beryl, emerald, synthetic emerald, Raman spectroscopy

By allowing positive identification of fluid and solid inclusions inside minerals, laser Raman microspectrometry has proved to be a perfect technique for discriminating between natural gems or synthetic minerals made for jewellery (sapphire, ruby, emerald)

opal, Mexico, gems

Gübelin provides a historical and technical survey of Mexican opal, discussing the principal varieties and their major localities. He notes the use of opal in pre-Columbian Mexico and describes the important deposits in the volcanic regions of Querétaro and Jalisco, where opal occurs in cavities and fractures in rhyolitic rocks. Particular attention is given to Mexican fire opal, with its characteristic yellow, orange and red body colors, as well as transparent precious opal displaying play-of-color. The article discusses the appearance, occurrence and characteristic inclusions of Mexican material; later studies cite Gübelin for two- and three-phase inclusions and mineral inclusions including hornblende, goethite, hematite, fluorite, quartz, cristobalite and chalcedony in Querétaro opal.

corundum, inclusions.5, gems

corundum, Australia.12, treatments.6, gems

Inclusions in sapphires from the New England gem fields of New South Wales are examined before and after commercial heat treatment to identify their mineral constituents and assess their usefulness in detecting treatment. Selected inclusions were photographed through polished surfaces, the stones were heated, and their surfaces were repolished before the same inclusions were photographed under comparable conditions. The stones were subsequently polished down to expose inclusions for major-element identification by scanning electron microscopy with energy-dispersive X-ray analysis. Heating was conducted at approximately 1700 °C in a gas–oxygen furnace, with the samples enclosed in porcelain and graphite crucibles alongside commercially treated rough. The principal observable change concerned rutile silk, which occurs as fine needles arranged in three sets intersecting at 120°, commonly concentrated in growth bands or distributed throughout the sapphire. Dense silk can give faceted stones a dull, greyish blue appearance. In a photographed example, a silky region that appeared pale honey-coloured in transmitted light became darker blue after heating, while its honey colour disappeared. The author attributes this change to the dissolution of rutile and suggests that the titanium released contributes to the blue colour, proposing a relationship to the development of blue colour in heated Sri Lankan geuda. Commercial heating generally removes visible silk, although some may survive when the original concentration is high or treatment conditions are insufficient. Fine silk therefore makes treatment less likely, but its absence does not establish that a stone has been heated, since naturally silk-free sapphires also occur. Other inclusions showed no discernible changes under the conditions studied. These included ruby-red to dark brown uranium pyrochlore crystals, sometimes accompanied by tension halos, healed fractures or comet-like trails; colourless feldspar inclusions, considered most likely plagioclase; opaque magnetite octahedra; and apparent negative crystals whose composition did not differ from that of the surrounding corundum. Strain around inclusions, twinning and fingerprint-like healed fractures likewise displayed no major differences. The study consequently identifies silk dissolution as the clearest observed internal response to heating while demonstrating the limited diagnostic value of the other features examined. Similarities between Australian inclusions and those reported from Thailand and Cambodia are related to their shared association with alkali basalts. The findings are presented as preliminary, with further work needed to develop reliable distinctions between heated and unheated Australian sapphires.

gems, fluorite, inclusions in fluorite, fluid inclusions

Several fluorite ± barite mineralizations, mainly controlled by post-Pliocene disjunctive faults, occur in silicified and altered Mesozoic limestones in the area of Termini Imerese (NW Sicily). In this study, we examined fluid inclusions in fluorite from the localities of Rocca Grande and Poggio Balate. Fluorite from both localities contains abundant two-phase liquid-rich aqueous inclusions. Less common types are: (a) monophase (liquid or gas); (b) two-phase, mostly gas-rich, hydrocarbon inclusions; (c) three-phase (liquid + gas + an accidentally trapped solid); and (d) three-phase (gas + two liquids: an aqueous brine and a hydrocarbonic fluid). In many inclusions, the presence of gas under pressure (most probably methane) is revealed by crushing tests. Homogenization temperatures (Th) of aqueous inclusions spread over a large range (60° to over 300 °C), being on average higher at Rocca Grande (∼200 °C) than at Poggio Balate (∼150 °C). Pressure corrections to Th are presumably minimal. However, several factors concur in suggesting that most Th’s are probably too high with respect to true trapping temperatures. Tentatively, the lowest Th-values (∼100 °C at Poggio Balate, and ∼150 °C at Rocca Grande) may be taken as indicative of trapping conditions. Temperatures of final melting of ice are, with few exceptions, higher than −3 °C. The geological setting and previous geochemical studies suggest that the fluorite-forming fluids were essentially heated meteoric waters. This is in agreement with the low salinities of most inclusions. On the other hand, reconnaissance δD determinations on bulk sample inclusions (‰ vs. SMOW) may reflect minor contributions of seawater to the hydrothermal fluids. The presence of abundant oil and methane is ascribed to interaction with organic matter-rich beds occurring in the local stratigraphic sequence.

corundum, inclusions.5, microscope, gems

The eyepiece pointer is a highly useful microscope accessory for the gemologist, as it remains in focus and consistently indicates the center of the microscope’s field of view regardless of magnification or the focal plane within the gemstone. It greatly simplifies the task of identifying and showing microscopic features in both mounted and unmounted gems, and it can be easily adapted for use with nearly any microscope.