Professor Dr Edward Joseph Gübelin

A tribute by E. Alan Jobbins, pp 257-259

HPHT treatment of different classes of type I brown diamonds

Thomas Hainschwang, Andrey Katrusha and Heiner Vollstaedt, pp 261-273

As part of a study of the classification and origin of colour of brown diamonds by one of the authors (Hainschwang, 2003), seven stones belonging to defined classes were selected and treated by the HPHT process. Each diamond was tested extensively before and after the treatment using standard and advanced gemmological methods, including FTIR spectroscopy, low temperature Vis/NIR and photoluminescence spectroscopy. On treatment, all diamonds except the CO₂ and ‘Pseudo CO₂’ diamonds lost their brown coloration and exhibited various shades of yellow. It was found that the spectra of all diamonds except CO₂ and ‘Pseudo CO₂’ diamonds showed very apparent changes of which the elimination of the amber centre with its primary peak between 4070 and 4165 cm^-1 in the FTIR spectra is of particular interest. The results of the treatment are discussed in detail and correlated with the proposed classification, of which a summary is presented.  One of the major results of this research is the identification of several origins of the brown colour in diamonds, including plastic deformation with associated defects and impurities such as CO₂.

Describing diamond beauty — assessing the optical performance of a diamond

Michael D. Cowing, FGA, pp 274-280

A diamond’s optical performance is the visual interaction of the diamond reflecting and refracting the surrounding light to the viewer. It is this performance of the diamond that results in its beauty. This work advances the idea that the best diamond cuts ‘evolved’ to have the highest optical performance in a variety of typical viewing and illumination circumstances. Current and historical descriptors of diamond beauty and performance are reviewed.   The rationale is put forward for the need to augment the current, single, quantitative measure of brilliance — light return intensity with the qualitative aspect of ‘contrast brilliance.’ Contrast brilliance is explained, and illustrated with diamond photography.  The relationship between contrast brilliance and scintillation is also examined.

Gem-quality musgravite from Sri Lanka

Dr Karl Schmetzer, Dr Lore Kiefert, Dr Heinz-Jürgen Bernhardt and Murray Burford, pp 281-289

Gemmological, chemical and spectroscopic properties of a transparent, gem-quality rough musgravite from Sri Lanka are presented.  A detailed analysis of musgravite reveals that applying X-ray diffraction and micro-Raman spectroscopy are decisive in establishing the identity of this rare gemstone. The greyish-blue pebble of 1.42 ct consists of a musgravite crystal with lamellar inclusions of spinel. With refractive indices of 1.721 (n_e) and 1.717 (n_o) and a specific gravity of 3.61, the musgravite has an iron content of 0.91 wt.% FeO, while the spinel exsolution lamellae reveal higher amounts of 1.89 wt.% FeO.

Iron- and zinc-rich gem-quality taaffeites from Sri Lanka

Dr Karl Schmetzer, Dr Lore Kiefert, Dr Heinz-Jürgen Bernhardt, Murray Burford and Dunil Palitha Gunasekara, pp 290-298

Gemmological, chemical and spectroscopic properties of a semitransparent, gem-quality rough (water-worn) taaffeite crystal of 2.52 ct and of two transparent, faceted taaffeites of 1.48 and 0.30 ct from Sri Lanka were determined. The rough crystal was identified by X-ray single crystal diffraction and Raman spectroscopy. By electron microprobe analysis an extraordinarily high value of 5.62 wt.% FeO was determined. This high iron content is consistent with the high refractive indices and the high specific gravity of the taaffeite crystal. The two faceted gemstones were identified by Raman spectroscopy and electron microprobe analysis which revealed extraordinarily high zinc values of 8.87 and 5.27 wt.% ZnO. These high zinc contents are responsible for the high refractive indices and the high specific gravities of both taaffeites.

Note on nephrite jade from Val Faller, Switzerland

Dr Douglas Nichol and Herbert Giess, pp 299-304

In the Oberhalbstein area of southeastern Switzerland, 12 occurrences of nephrite jade are recorded in intimate association with serpentinized peridotite within an alpine ophiolite complex of Mesozoic age. These include two roadside localities in Val Faller that have been enlarged as a result of highway improvement schemes through the valley. Fresh rocks exposed in new rock cuttings reveal the contact alteration zones between serpentinites and the country rocks. The small pods and irregular patches of nephrite jade are classed as ortho-nephrite in type. Predominant colours are of uneven greyish-green hue, grain size varies from fine- to coarse-grained and surface lustre is vitreous. The nephrite jade formed as a result of metasomatism and high-pressure effects following the emplacement of the ultrabasic rocks and serpentinization. Although the occurrences of nephrite are of insufficient size to warrant mining operations, the sites in Val Faller have considerable historical significance and remain popular with collectors of mineral specimens.

Nephrite jade from Mastabia in Val Malenco, Italy

Douglas Nichol and Herbert Giess, pp 305-311

At Mastabia in Val Malenco, northern Italy, a major deposit of nephrite jade was recently identified at the site of an abandoned talc mine. Geologically, the talc orebody comprises a series of steeply dipping lenses associated with a fault wedge of dolomitic marble and calc-silicate rocks of Triassic age. The nephrite jade is hosted within massive tremolitite in the central core of the talc orebody and the nephrite and tremolitite were both discarded as waste material during talc mining operations. Recovery of nephrite from the spoil heaps is now underway. Predominant colours of the nephrite jade are of uneven pale green and yellow-green hue. Grain size ranges from fine- to coarse-grained and accessory constituents include calcite, talc and opaque iron minerals. The nephrite jade is classified as para-nephrite in type and formed together with talc and tremolitite by intense hydrothermal alteration and decarbonation of dolomitic marble along shear zones related to thrust structures. The quantity of nephrite jade available remains uncertain but appears substantial. The nephrite is fashioned into a range of jewellery and ornamental pieces.

Stereoscopic effect in asterism and chatoyancy

Harold Killingback FGA, pp 312-315

The optical effect of a star displayed by rose quartz which appears to be located above the stone is described.  An explanation of the phenomenon using ray diagrams is proposed.  This analysis applies to all star and cat’s-eye stones.

Identification of an imitation pearl by FTIR, EDXRF and SEM

T.L. Tan, T.S. Tay, S.K. Khairoman and Y.C. Low, pp 316-324

Pearl is an organic gem and its popularity is basically due to the beauty of its natural lustrous and iridescent surface. Most of the pearls in the market are now grown ('cultured') in farms containing thousands of molluscs. Each mollusc deposits a nacreous layer on an inserted bead of mother-of-pearl material. For superior quality pearls, it may take up to two years or more to harvest and the yield is usually low, and these factors account for their high price and the proliferation of pearl imitations. Pearl imitations whicha re devloped in laboratories are not easily identified using conventional gemmological methods. In this investigation, the surface structures of the cultured pearl and its imitations were studied using a scanning electron microscope (SEM) at magnification of up to 2000x. Energy-dispersive X-ray fluorescence (EDXRF) and Fourier transform infrared (FTIR) spectroscopy were employed to study the chemical composition of the pearl layer and the bead for naturally-coloured cultured pearls, dyed cultured pearls and pearl imitations. EDXRF results show that the cultured pearls and their beads are basically CaCO₃, while the detection of only C and O in the pearl imitations indicates that they are consistent with a polymeric compositon. The presence of Si, O, Na and Al in the bead of the pearl imitation is consistent with it being glass. In FTIR experiments, the absorption peaks of 700 cm^-1, 713 cm^-1, 862 cm^-1 and 1083 cm^-1 are observed in both naturally-coloured and dyed cultured pearls, which confirm that they are CaCO₃ with aragonite structure. Howeve,r the infrared spectra of pearl imitations are very different and are typical of polymers. In addition, studies on fluorescence using short and long wave ultraviolet radiation indicate that cultured pearls can be effectively distinguished from their imitations.

X-ray luminescence, a valuable test in pearl identification

Professor Dr H.A. Hänni, Dr L. Kiefert and P. Giese, pp 325-329

The increasing similarity of structures encountered in natural pearls and beadless freshwater cultured pearls requires one or more additional criteria for their differentiation.  The majority of natural pearls are from saltwater oysters; in contrast, most beadless cultured pearls come from freshwater mussels.  For some time it has been known that freshwater pearls produce luminescence under X-rays, whereas pearls grown in saltwater do not.  The reason is because freshwater nacre contains traces of manganese.  By using a sensitive camera this visible luminescence can be recorded and displayed on a monitor.  The beads (from freshwater nacre) in Japanese saltwater cultured pearls (Akoya) also react to the X-ray excitation and may shine through the cultured overgrowths that are relatively thin.  The method is used as an additional test and is not an alternative for X-radiograph images.

The variation of RI with rotation of a doubly refractive gemstone on the refractometer hemicylinder

Xinhua Song, Ruihua Wu and Weizheng Wu, pp 331-340

The hemicylinder refractometer is used to determine the refractive index (RI) of faceted anisotropic gemstones as a function of crystal orientation. The technique is difficult due to large errors in measurement and uncertainty in the orientation of facets. The theoretical variation is investigated using analytical geometry to generate comprehensive parametric graphs of RI variation. These graphs are analysed systematically, providing methods with reference examples for determining the optic character of anisotropic gemstones.

Use of the polarizing filter on the refractometer

B. Darko Sturman, pp 341-340

Use of the refractometer is summarized and a new comprehensive method to obtain reliable refractive indices is described. A single table contains all possible combinations of RI data and should be kept with the gemmological refractometer for practical use by both beginners and expert gemmologists.