gems, corundum, treatments.6, inclusions.5, Madagascar.12, pink sapphire

Rutile, titanium dioxide (TiO₂), is a tetragonal mineral that generally occurs in corundum as silk (exsolution-formed needles), dust-like exsolution particles, or macroscopic crystal inclusions. Rutile crystals can have orange to deep red-brown coloration, and the crystal shapes can be rounded or euhedral. The author recently examined a 4.24 ct faceted transparent blue sapphire from Afghanistan. The stone contained dusty flake-like inclusions, naturally healed fractures, lathe-like inclusions, twinning, rounded colorless crystals identified as apatite (RRUFF R040098), and an unusual long prismatic orange crystal identified as rutile (RRUFF R040049). This rutile crystal shape (figure 12) is not typical for corundum, particularly in blue sapphire.

gems, corundum, inclusions.5, Mozambique.12, Burma.12

The red variety of corundum owes its color and strong fluorescence to the presence of Cr, as well as traces of Fe. The latter can reduce the fluorescence and thus impact the appearance of the final gem. Gem quality rubies are rarely available for scientific study and even less common in their rough form. Opaque inclusions in rubies are often removed during faceting and remain unidentified. This study aims to identify opaque inclusions in rubies from the two most common origins seen in the high end market today: Mogok, Myanmar and Montepuez, Mozambique. Using electron probe microanalaysis (EPMA) the inclusions were identified as sphalerite and pyrrhotite in Mogok rubies. The paragenesis of Myanmar, marble-related rubies is fairly well understood and no Fe-rich minerals apart from sulfides have been identified. Opaque inclusions in Mozambican rubies are a complex mix of Fe-Cu-Ni sulfides with exsolution textures. These inclusions are interpreted to be small amounts of sulfide melt trapped during corundum formation. The different sulfide phases crystallized from this entrapped melt and some phases experienced later exsolution during cooling. The formation of amphibole-related, Mozambican rubies is not well understood, but it is obvious that very different processes are at work compared to the marble-related Myanmar ruby deposits.

gems, diamond, corundum, inclusions

gems, corundum, inclusions.5

A phenomenon resembling play-of-color was seen in a 4.13 ct unheated sapphire recently examined by the author. When tilted in darkfield lighting (watch the video below), the sapphire displayed this noteworthy effect, which is most commonly seen in precious opal. It occurred within a small section of a single fine, milky cloud with an unusual shape (figure 1, left). The cloud also possessed extensive transparent graining confined within its borders, best seen in brightfield illumination (figure 1, right). Some regions of the cloud displayed an appearance of rainbow graining with spectral colors confined to the linear grain lines and looked one-dimensional. Other regions, however, expressed a soft billowy broad flash of altering colors with diffuse edges resembling play-of-color and did not follow linear grain lines (figure 2). Play-of-color in opal is defined as a display of spectral colors due to the diffraction of light as it passes through organized, submicroscopic spherical particles. It is possible that the combination of fine milky particles and abundant transparent graining within the cloud were able to diffract light in a way that resembles play-of-color.

gems, beryl, emerald, Colombia, Afghanistan, Brazil, Russia, Mozambique, Madagascar, Zambia, Zimbabwe, India, geology, analytical, provenance, emerald worldwide

Although emerald deposits are relatively rare, they form in specific yet varied geological settings, and current classification models are often too restrictive, leading to confusion about the precise formation mechanisms of certain occurrences; emerald is essentially beryl containing sufficient concentrations of chromium and vanadium to produce its characteristic green hues, and its formation depends on conditions supplying both beryllium and chromium or vanadium—traditionally grouped into three types: desilicated pegmatite-related deposits formed by metasomatic fluid interaction with beryllium-rich pegmatites intruding Cr- or V-rich rocks; sedimentary deposits, responsible for much gem-quality emerald, formed by brine interaction with Cr/V-bearing sedimentary rocks along faults; and the rarer metamorphic-metasomatic deposits involving deep crustal fluids and metamorphosed shales, carbonates, and ultramafic rocks—with all three models requiring tectonic activity, which can transform one type into another through metamorphism and partial melting, thus blurring distinctions; to address the limitations, this paper proposes an enhanced classification system based on geological environment (magmatic or metamorphic), host-rock type, metamorphic grade, mineralization style, and fluid characteristics, accounting for multi-stage processes, beryllium remobilization, and using chemical, isotopic, and fluid inclusion studies to support genetic models, thereby categorizing emeralds into two main types: Type I (tectonic magmatic-related) with sub-types in (IA) mafic-ultramafic rocks (e.g. Brazil, Zambia, Russia), (IB) sedimentary rocks (e.g. China, Canada, Norway), and (IC) granitic rocks (e.g. Nigeria); and Type II (tectonic metamorphic-related) with sub-types in (IIA) mafic-ultramafic rocks (e.g. Brazil, Austria), (IIB) sedimentary rocks (e.g. Colombia, Canada, USA), (IIC) metamorphic rocks (e.g. China, Afghanistan, USA), and (IID) metamorphosed and remobilized or hidden intrusion-related deposits (e.g. Austria, Egypt, Australia, Pakistan), plus some unclassified occurrences.

gems, corundum, ruby, origin determination, Burma.12, Mozambique.12, Madagascar.12, Kenya.12, Tajikistan.12, Vietnam.12, Afghanistan.12

Over the last several decades, geographic origin determination for fine rubies has become increasingly important in the gem trade. In the gemological laboratory, rubies are generally broken down into two groups based on their trace element chemistry: marble-hosted (low-iron) rubies and high-iron rubies. High-iron rubies are usually a straightforward identification based on their inclusions and trace element profiles. Marble-hosted rubies can be more challenging, with some deposits showing overlap in some of their inclusion scenes. But many marblehosted rubies, especially Burmese stones from Mogok and Mong Hsu, can be accurately identified based on their internal features and trace element profiles. This contribution will outline the methods and criteria used at GIA for geographic origin determination for ruby.

gems, chrysoberyl, alexandrite, Russia, Brazil, Sri Lanka, India, Tanzania

The gem and jewelry trade has come to place increasing importance on the geographic origin of alexandrite, as it can have a significant impact on value. Alexandrites from Russia and Brazil are usually more highly valued than those from other countries. In 2016, GIA began researching geographic origin of alexandrite with the intent of offering origin determination as a laboratory service. Unfortunately, collecting reliable samples with known provenance can be very difficult. Alexandrite is often recovered as a byproduct of mining for other gemstones (e.g., emerald and corundum), so it can be difficult to secure reliable parcels of samples because production is typically erratic and unpredictable. The reference materials studied here were examined thoroughly for their trace element chemistry profiles, characteristic color-change ranges under daylight-equivalent and incandescent illumination, and inclusion scenes. The data obtained so far allow us to accurately determine geographic origin for alexandrites from Russia, Brazil, Sri Lanka, Tanzania, and India. Future work may help to differentiate alexandrites from other localities.

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

Since 2015, a large quantity of a new blue diffused synthetic sapphire has appeared in the Chanthaburi market. It is interesting to compare its gemmological properties with those of a blue diffused natural sapphire that came onto the market in 2012. In immersion, both types of blue diffused sapphires showed the “spiders’ web effect” — a cut-related colouration pattern — and rather high titanium content on the surface of the samples, both effects being diagnostic evidence of Ti-diffusion treatment. The synthetic test stone, however, also displayed strong chalky blue fluorescence to SWUV radiation (also in the DiamondView™), the presence of curved bands, minute particles or gas bubbles under a microscope, very low Ga contents, and no Fe³⁺-related absorption peaks on the UV–Vis spectra. In contrast, the natural one showed the occurrence of tension discs, altered solid inclusions and altered fingerprints, often with colour concentration along healed fractures, cavities seen under the microscope, significant Ga content, and some Fe³⁺-related absorption peaks on the UV–Vis spectra. The thickness of the blue colour rim of the diffused natural stone was apparently much thicker than that of the diffused synthetic one. The reason is that it is the penetration depth of titanium that controls the colour rim thickness of the natural stone, while it is the penetration depth of iron that controls that of the synthetic one. Because of the difference in colour rim thickness and the repolishing process, the “spiders’ web effect” in the natural stone is less pronounced, while that in the synthetic one is more obvious. The formation of the blue colour rim and multi-element diffusion found in both synthetic and natural sapphires are also discussed here.

gems, corundum, ruby, Thailand.12, Bo Rai

The Bo Rai alluvial gem field in Trat Province, eastern Thailand, was a major mining site for Thai ruby during the early 1980s, and this material continues to circulate in the gem market worldwide. For this study, approximately 1,000 Bo Rai ruby samples were examined and pre-screened for mineral inclusions. The rough stones usually formed platy, waterworn, tabular crystals with etched or resorbed surfaces. UV-Vis-NIR spectroscopy indicated a relatively high Fe content, which was confirmed by trace-element analysis. Solid inclusions typically consisted of Al-rich pyroxene, plagioclase and pyrope with subordinate sillimanite and spinel (both of which are reported here for the first time), as well as sulphides and silicate melt inclusions. The inclusion assemblage of pyroxene, plagioclase, pyrope and spinel closely resembles the mineralogy of mafic granulite xenoliths in alkali basalt associated with the Bo Rai gem field, which supports ruby formation in mafic granulite prior to being transported to the earth’s surface via basaltic eruptions.

gems, corundum, Australia.12, Cambodia.12, China.12, Thailand.12, Nigeria.12, inclusions.5, treatments.6, basalt, sapphire

Basalt-related blue sapphires typically have an oversaturated blue color causing the stones to be too dark when faceted. Low-temperature heat treatment can lighten dark material. However, it is difficult to identify this treatment using standard gemological testing and microscopic examination. For instance, unheated sapphires are transported to the earth’s surface by hot, basaltic magma which may have thermally altered their inclusions, causing them to look similar to those seen in artificially heated material. In this study, 50 basalt-related blue sapphires were heat treated in air (oxidizing atmosphere). Changes in color appearance, UV fluorescence, internal features, and spectroscopic properties post treatment were compared with the results before treatment to find suitable approaches for detecting heat treatment on blue sapphires from basalt-related deposits. The main factors affecting the sapphires’ response to heat treatment include the temperatures employed and the duration of the treatment process. Initial experiments at temperatures of 500–1500 °C for 7 hours showed that the blue color appears to lighten at 700 °C and higher. Temperatures ranging from 700 to 1050 °C were then selected for a more in-depth study on the effect of heating time with durations of 1.75, 7, and 28 hours. The results showed negligible change to a slight lightening of the blue color after treatment at 700 °C, while an obvious lightening of the blue color occurred at 900 and 1050 °C. Some mineral inclusions, iron stains, and partially healed fractures showed signs of alteration during these heating experiments, while needles and minute particles did not show any signs of change. All unheated samples studied were inert to short-wave ultraviolet radiation. After heating, most remained inert under short-wave UV, but a few samples exhibited a very weak chalky green fluorescence. FTIR spectra obtained from the samples varied considerably, with differences in peak intensities before and after heating. Differences were also seen as the duration of heating was changed. In most cases, unheated blue sapphires from basalt-related deposits revealed the characteristic 3309 cm⁻¹ series (sharp peaks at 3309, 3232 and 3186 cm⁻¹) in the FTIR spectrum with the intensity of 3232 cm⁻¹ being much lower than that of 3309 cm⁻¹. After heating at 700 °C and 900 °C, the intensity of the 3309 cm⁻¹ peak decreased and the 3232 cm⁻¹ peak increased, respectively. However, in most cases, some samples that initially showed a relatively intense 3232 cm⁻¹ peak before treatment exhibited a more intense 3309 cm⁻¹ peak and a less intense 3232 cm⁻¹ peak post treatment. After heating at 1050 °C for 1.75 hours, we recorded a decrease in the intensity of the 3309 cm⁻¹ peak and an increase in the 3232 cm⁻¹ peak, to the point where they were of almost comparable intensity. When subsequently heated for 7 and 28 hours, the intensity of the 3309 cm⁻¹ peak increased and that of the 3232 cm⁻¹ peak decreased. UV-Vis-NIR spectra obtained from the samples heated at 900 °C and 1050 °C for 1.75, 7, and 28 hours showed a reduction in the height/intensity of the broad band centered at 580 nm related to an Fe²⁺–Ti⁴⁺ intervalence charge transfer. This reduction is the root cause of the lighter color after heat treatment. The results of this study showed that even with advanced testing, it is very challenging to separate basalt-related sapphires that have been artificially heated from natural unheated stones, owing to the variable results associated with the experimental temperatures and durations used. Inclusion studies may provide sufficient evidence in some cases, but even when comparing the inclusions seen before and after treatment, separation often remains challenging.