Given below are abstracts of articles published in The Journal of Gemmology, 2008, 31(1-2). This volume is available online for current Gem-A members with full search facilities.
A water-worn pebble of good clarity purchased in Bangkok started the quest that identified this as gem-quality thortveitite, a scandium yttrium silicate. A faceted stone and a cleavage fragment, both from the original pebble, have been tested. Results from standard gemmological tests on the faceted stone did not match those from any known gemstone. The stone and fragment were subsequently identified as high-scandium thortveitite. The unusual chemistry indicates a possible synthetic origin; however, a natural origin is suggested by the presence of three-phase inclusions in a planar array (feather) within the faceted stone.
This review provides an overview of numerous patent documents that have been published recently and describe various coating technologies for gem materials. Parts of the documents focus on the deposition of one single or several specifically structured or composed layers in order to obtain the desired colour and the desired optical effects or wear resistance. Contact heat treatment of faceted stones with solid transition metal-bearing plates as well as heat treatment of layers deposited on the surfaces of the gemstones, are also reported. Chemical data of one sample of iron-coated topaz provide a first insight into the reaction mechanism within the outer diffusion layer.
Computerized three-dimensional X-ray microtomography is capable of revealing the internal structure and some of the material properties of pearls at high resolution, and is thus a more useful tool in pearl testing than conventional X-ray radiography. Differences in transparency to X-rays (radiodensities) can be detected and speed up the process of locating different CaCO3-polymorphs in the pearl. When combined with micro-Raman spectroscopy, vaterite-containing areas can be identified.
Specular reflectance FTIR (Fourier Transform Infrared) spectroscopy is a relatively little-exploited but very efficient technique to identify minerals and other materials that possess a somewhat reflective surface. In the past years the Gemlab and GemTechLab laboratories have used and refined the method for gemmological purposes and developed an extensive database of specular reflectance infrared spectra of minerals, gemstone imitations, organic materials and synthetic gem materials. This paper explains the method, introduces the spectrometer and accessories used and highlights the practical applications and advantages of specular reflectance FTIR spectroscopy in gemmology.
Colour varieties of Afghan beryls from pegmatites at Ghursalak in Konar Province (aquamarine, morganite) and from the Panjshir Valley (emerald) have been investigated. The aquamarine, composition with SiO2 (65.75 wt.%), Al2O3 (17.87 wt.%) and BeO (12.29 wt.%) is similar to that of many other aquamarines, and the low concentration of alkalis as well as relatively high amounts of Li2 O (0.34 wt.%) all indicate the gemstone to be normal beryl with limited isomorphous substitution in octahedral and tetrahedral sites. The calculated unit cell parameters (i.e. a = 9.2221 Å, c = 9.1990 Å, c/a = 0.9975), optical characteristics (i.e. ω = 1.576, ε = 1.571; Δ = 0.005) and the IR spectra of aquamarine with characteristic bands at 1206 and 963 cm-1 are typical of normal beryl. Emerald with contents of SiO2 (64.50 wt.%), Al2O3 (15.19 wt.%) and BeO (12.73 wt.%) and significant amounts of alkalis (i.e. Na2O+K2O+Li2O = 2.1 wt.%) is alkali-rich octahedral beryl. This attribution is also indicated by the unit-cell parameters (i.e. a = 9.2399 Å, c = 9.1984 Å, c/a = 0.9955) and optical characterization (i.e. ω = 1.580, ε = 1.574; Δ = 0.006). Morganite with SiO2 (64.16 wt.%), Al2O3 (17.91 wt.%) and BeO (11.81 wt.%) has the highest total alkali content (Na2O+K2O+Li2O = 3.51 wt.%) and is assigned to alkali-rich (Na-K-Li) tetrahedral beryl. The IR spectra of morganite (additional band at c. 1060 cm-1, no shifts of 1215, 970 cm-1 towards lower wave numbers) together with calculated unit-cell parameters (i.e. a = 9.2198 Å, c = 9.2314 Å, c/a = 1.0012) and optical characteristics (i.e. ω = 1.580, ε = 1.574;
Δ = 0.006) are all typical for a tetrahedral beryl. The Afghan beryls formed within granitic, beryl-muscovite pegmatites in hydrothermal conditions.
An explanation is offered of the apparent difference between the positions of light spots on a star rose quartz sphere as seen in a photograph of the actual sphere, compared with a photo of a table tennis ball marked with the orientations of these spots.
The rare gems taaffeite and musgravite have lately become more popular among collectors. Due to their similar chemical compositions and crystal structures, their main gemmological properties overlap and so sophisticated measurement techniques such as quantitative chemical analysis, Raman spectroscopy or X-ray powder or single crystal diffraction are needed for their identification. This study describes an EDXRF instrument used as single-crystal X-ray diffraction apparatus and the technique to identify taaffeite and musgravite in a relatively conclusive procedure. A special rotating and tilting stage has been constructed to non-destructively determine the differences in diffraction pattern based on the different symmetries (trigonal and hexagonal), unit cell dimensions and space groups of taaffeite and musgravite.