Given below are abstracts of articles published in The Journal of Gemmology, 2008, 31(3-4). This volume is available online for current Gem-A members with full search facilities.

A fancy reddish brown diamond with new optical absorption features

Taijin Lu, Tatsuya Odaki, Kazuyoshi Yasunaga and Hajime Uesugi.  pp 73-76

Red or near-red natural diamonds are extremely rare, and the possible origins of their colour are unclear. A weak absorption band at 776 nm in the visible range and a clear absorption band at 6169 cm^-1 in the infrared region were detected in a near-red diamond, which could be factors in understanding the possible origin of this colour in natural diamonds.

A place for CZ masters in diamond colour grading

Michael D. Cowing. pp 73-83

Over the last 20 years and despite recommendations to the contrary, many gemmologists and appraisers have gravitated to the use of cubic zirconia (CZ) master stone sets to assist in the colour grading of diamonds. This investigation revisits with new insight, diamond grading technique and methodology. It addresses the judicious use of CZ master stone sets to augment diamond masters that are smaller in size and number. Study results support the use of accurately graded, carat-size CZs in reducing the subjectivity of colour grading when only incomplete (every other grade) diamond master sets of small (under 0.4 ct) sizes are available.

The geological context of gems in the Velasco Pegmatitic District, Argentina

Fernando Guillermo Sardi. pp 85-89

The Velasco Pegmatitic District is located in the Velasco range, La Rioja province, northwestern Argentina. The Velasco range is made up of several granitic units of different petrography, magmatic evolution and age among which are the Huaco and Sanagasta granites. The pegmatites in the Velasco district have a spatial and genetic relationship with these granites and belong to the rare-element class, beryl type, beryl-columbite-phosphate subtype. The pegmatites are usually zoned and the K-feldspar zone is generally the most important for gem-quality beryl. Heliodor and aquamarine varieties are present and these crystals can be cut to yield products of attractive beauty. Other beryl crystals with poor transparency can be utilized for cabochons or tumble polished. Rock-crystal and rose-quartz are also present in some pegmatites from the Velasco district.

Magnetic susceptibility,  a better approach to defining garnets

Dr D. B. Hoover FGA FGAA (Hon.), C. Williams FGA, B. Williams and C. Mitchell FGA. pp 91-103

Using a new, non-destructive method of gem testing, magnetic susceptibility, the authors show how the major end-member composition of any garnet may be confidently predicted by plotting RI against measured susceptibility. On this diagram, eight end-member garnets are plotted, so that any measured garnet can be placed in an appropriate ternary area. This method shows how previous methods of identifying garnets — by their colour, RI and spectrum — are insufficient to accurately identify chemistry in the garnet group. Furthermore, it can be done with inexpensive equipment available to most gemmologists.

The refraction of light by garnet depends on both composition and structure

David K. Teertstra. pp 105-110

The index of refraction (n), used to identify minerals, depends on both composition and crystal structure. The method of optical analysis implies that a photon refracts by local interaction with individual ions. The index n is calculated exactly using a dependence of the specific refractivity of ions on the inter-ionic distances and angles of the crystal, and is influenced by bonds to surrounding ions up to half-a-unit-cell distant.

Ornamental variscite: A new gemstone resource from Western Australia

Margot Willing, Susan Stöcklmayer and Dr Martin Wells. pp 111-24

Variscite (AlPO₄.2H₂O) is an uncommon bluish-green mineral that has been used as an ornamental gemstone since the Neolithic Age. A new location of variscite has recently been investigated on Woodlands Station, Western Australia. The variscite from here occurs in a variety of aggregated habits within fine-grained Proterozoic siltstones and breccias. Rock surfaces have been found encrusted with microcrystals of variscite and wardite. Within the host rocks, associated minerals include metavariscite, crandallite, jarosite, quartz and grains of native gold. The gemmological properties of the Woodlands variscite are: RI range of 1.570 to 1.582, and SG from 2.49 to 2.55, and these are within the ranges of variscite from other world sources. X-ray diffraction analysis confirms the variscite to be of the ‘Meßbach-type’ in association with metavariscite, a dimorph of variscite. Using visible reflectance spectroscopy, trivalent chromium was verified as the main chromophore responsible for the colour. Minor vanadium is also present, probably in mixed oxidation states of +3 and +4.

Visually distinguishing A-jadeite from B-jadeite

Li Jianjun, Liu Xiaowei, Zhang Zhiguo, Luo Yueping, Cheng Youfa and Liu Huafeng. pp 125-31

Jadeite grades A and B are described in the context of the Chinese gem trade. Visual distinction of the grades is described, illustrated and discussed in terms of surface lustre, distribution of colour, nature of inclusions, internal reflection of light, microfractures and features of the fashioning of various items. Development of these recognition skills is recommended but recourse to the laboratory should always be an option for difficult items.