Coexisting Rubies and Blue Sapphires from Major World Deposits: A Brief Review of Their Mineralogical Properties

Gem corundum deposits are typically divided into blue sapphire and ruby deposits. However, this classification often overlooks the fact that the precious stones produced are the same mineral with only an overall slight difference in their trace element profiles. It can take only a couple thousand ppm chromium to create the rich, red color expected of a ruby. This contribution deals specifically with economically important gem corundum mining regions that produce both blue sapphires and rubies either in comparable quantities (Mogok, Myanmar, and the basalt-related gem fields on the border between Thailand and Cambodia at Chanthaburi, Thailand, and Pailin, Cambodia) or predominantly blue sapphires with rare rubies (secondary Montana sapphire deposits and Yogo Gulch in Montana as well as the gem fields of Sri Lanka). Comparison of the trace element profiles and inclusions in the blue sapphire/ruby assemblages in these deposits shows that there are both monogenetic and polygenetic assemblages in which the blue sapphires and rubies have the same geological origin (monogenetic) or distinct geological origins (polygenetic). In the monogenetic assemblages, the rubies and blue sapphires have essentially indistinguishable inclusions and trace element chemistry profiles (with the exception of Cr contents). On the other hand, polygenetic assemblages are composed of rubies and blue sapphires with distinct inclusions and trace element chemistry profiles. Notably, in the monogenetic assemblages, chromium seems to vary independently from other trace elements. In these assemblages, Cr can vary by nearly four orders of magnitude with essentially no consistent relationship to other trace elements. The observations described herein are an attempt to address the question of what the geochemical and geological constraints are that turn gem corundum into a spectacular ruby

The origin of needle-like rutile inclusions in natural gem corundum: a combined EPMA, LA-ICP-MS, and nanoSIMS investigation

Trace-element chemistry and microscopic observations of included gem corundum (α-AlO) suggests a new model of syngenetic growth of oriented rutile inclusions rather than the usual interpretation of their growth through exsolution. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is now a robust method for measuring trace elements in gem-quality corundum (ruby and sapphire). Nonetheless, the corundum structure is relatively unforgiving for substitutional components and typically only a small handful of minor to trace elements are measured by LA-ICP-MS (Mg, Ti, V, Cr, Fe, Ga). Less commonly, trace elements such as Be, Zr, Nb, Sn, La, Ce, Ta, and W are found in natural corundum. Their concentrations are typically correlated with high contents of Ti and silky or cloudy zones in the corundum that contain a high concentration of needle-like rutile or other oxide inclusions. Three metamorphic-type sapphires from Sri Lanka, Madagascar, and Tanzania were studied here using LA-ICP-MS, electronprobe microanalysis (EPMA), and nanoSIMS to document correlations between the various trace elements and their distribution between the corundum and included, oriented rutile TiO needles. NanoSIMS and EPMA measurements show concentration of Be, Mg, Fe, V, Zr, Nb, Ce, Ta, and W in the rutile needles. The relative atomic concentrations of Mg and Ti from LA-ICP-MS measurements suggest the corundum-rutile intergrowth grew as a mechanical mixture of the two phases as opposed to rutile formation through exsolution from the corundum host. This scenario is also suggested for the three magmatic-type sapphires studied here based on the presence of glassy melt inclusions in close association with included, oriented oxide needles. The preservation of a glassy melt inclusion requires fast cooling, whereas exsolution of the oxide inclusions would require slow cooling and annealing at a temperature lower than sapphire formation. The studied sapphires suggest the likely origin of the oriented, needle-like rutile inclusions to be syngenetic epitaxial coprecipitation of both rutile and corundum. The interpretation of such oriented oxide inclusions has important implications for understanding the geological formation conditions based on trace element data or using such data to separate sapphires and rubies based on their geographic origin

How to facet gem-quality chrysoberyl: clues from the relationship between color and pleochroism, with spectroscopic analysis and colorimetric parameters

Pleochroism plays an important role in determining the face-up visual color appearance of faceted, optically anisotropic (non-cubic) gemstones. One area that has received little attention is the interplay between pleochroism and the so-called alexandrite effect wherein the perceived color of a mineral changes with different lighting conditions (i.e., daylight vs. incandescent light). In this article we have collected ultraviolet/visible/near-infrared (UV-Vis-NIR) spectra of a gem-quality, synthetic Cr-bearing chrysoberyl crystal along its three crystallographic axes. We use these spectra to calculate the color and to quantify the color change that would be observed in a wafer or faceted gemstone in any orientation and for any prescribed path length of light between 1 and 25 mm. We describe the method used to perform these calculations and give an overview of color science and color space as it pertains to mineralogy and gemology. The data collected here are used to predict the optimum orientation for a wafer or a faceted alexandrite gemstone to produce the maximum color change sensation between daylight and an incandescent light source. We find that a wafer oriented with the unpolarized light-path-length perpendicular to the a-axis exhibits the strongest color change but that the color change is weaker parallel to the a-axis. Pleochroism in a faceted stone will mix light traveling in different directions. This relaxes requirements to orient a stone along the “best” direction, but it is still found that stones cut with their table to culet direction oriented perpendicular to the a-axis show the best color-change while orientation parallel to the a-axis produces weaker color change. Nonetheless, there is a wide range of “acceptable” orientations and no single “best” direction for a faceted gemstone. The results of this study demonstrate the complex nature of color in minerals and shed light on the intricate interplay between several factors including pleochroism, lighting conditions, light path length through a transparent sample, and chromophore concentrations. The use of the techniques outlined here can lead to a better understanding of the color sciences in the mineral world in general

Solid-state NMR and short-range order in crystalline oxides and silicates: a new tool in paramagnetic resonances

Most applications of high-resolution NMR to questions of short-range order/disorder in inorganic materials have been made in systems where ions with unpaired electron spins are of negligible concentration, with structural information extracted primarily from chemical shifts, quadrupolar coupling parameters, and nuclear dipolar couplings. In some cases, however, the often-large additional resonance shifts caused by interactions between unpaired electron and nuclear spins can provide unique new structural information in materials with contents of paramagnetic cations ranging from hundreds of ppm to several per cent and even higher. In this brief review we focus on recent work on silicate, phosphate, and oxide materials with relatively low concentrations of paramagnetic ions, where spectral resolution can remain high enough to distinguish interactions between NMR-observed nuclides and one or more magnetic neighbors in different bonding configurations in the first, second, and even farther cation shells. We illustrate the types of information available, some of the limitations of this approach, and the great prospects for future experimental and theoretical work in this field. We give examples for the effects of paramagnetic transition metal, lanthanide, and actinide cation substitutions in simple oxides, pyrochlore, zircon, monazite, olivine, garnet, pyrochlores, and olivine structures.In many oxide and silicate materials containing paramagnetic components at the hundreds of ppm to many percent level, high-resolution solid-state NMR spectra can provide important new types of information about short-range cation order/disorder, through often-large effects of unpaired electron spins on nuclear spins

A common origin for Thai/Cambodian rubies and blue and violet sapphires from Yogo Gulch, Montana, U.S.A.?

A wide number of genetic models have been proposed for volcanically transported ruby and sapphire deposits around the world. In this contribution we compare the trace element chemistry, mineral and melt inclusions, and oxygen isotope ratios in blue to reddish-violet sapphires from Yogo Gulch, Montana, U.S.A., with rubies from the Chantaburi-Trat region of Thailand and the Pailin region of Cambodia. The similarities between Thai/Cambodian rubies and Yogo sapphires suggest a common origin for gem corundum from both deposits. Specifically, we advance a model whereby sapphires and rubies formed through a peritectic melting reaction when the lamprophyre or basalts that transported the gem corundum to the surface partially melted Al-rich lower crustal rocks. Furthermore, we suggest the protolith of the rubies and sapphires was an anorthosite or, in the case of Thai/Cambodian rubies, an anorthosite subjected to higher pressures and converted into a garnet-clinopyroxenite. In this model the rubies and sapphires are rightfully considered to be xenocrysts in their host basalts or lamprophyre; however, in this scenario they are not "accidental" xenocrysts but their formation is intimately and directly linked to the magmas that transported them to the surface. The similarities in these gem corundum deposits suggests that the partial melting, non-accidental xenocryst model may be more wide-reaching and globally important than previously realized. Importantly, in both cases the gem corundum has an ostensibly "metamorphic" trace element signature, whereas the presence of silicate melt (or magma) inclusions shows they ought to be considered to be "magmatic" rubies and sapphires. This discrepancy suggests that existing trace element discriminant diagrams intended to separate "metamorphic" from "magmatic" gem corundum ought to be used with caution

Raman and Photoluminescence Mapping of Gem Materials

Raman and photoluminescence (PL) mapping is a non-destructive method which allows gemologists and scientists to evaluate the spatial distributions of defects within a gem; it can also provide a method to quickly distinguish different species within a composite gem. This article provides a summary of this relatively new technology and its instrumentation. Additionally, we provide a compilation of new data for various applications on several gemstones. Spatial differences within diamonds can be explored using PL mapping, such as radiation stains observed on the rough surface of natural green diamonds. Raman mapping has proven useful in distinguishing between omphacite and jadeite within a composite of these two minerals, identifying various tourmaline species within a heterogeneous mixture, and determining the calcium carbonate polymorphs in pearls. Additionally, it has potential to be useful for country-of-origin determination in blue sapphires and micro-inclusion analysis. As new avenues of research are explored, more applications for gem materials will inevitably be discovered