Mexican Gem Opals The Magnetic Index Spectroscopic Properties of Möng Hsu Ruby Interesting American Opals: Wood Replacement Louisiana Opal Gemmological Study of Corundum from Madagascar Recent Observations of Composite Stones

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MEXICAN GEM OPALS
NANO- AND MICRO-STRUCTURE, ORIGIN OF COLOUR,
COMPARISON WITH OTHER COMMON OPALS OF GEMMOLOGICAL SIGNIFICANCE

E. Fritsch¹, M. Ostrooumov², B. Rondeau³, A. Barreau¹, D. Albertini¹, A.-M. Marie¹,
B. Lasnier⁴, J. Wery¹

1 Institut des Materiaux Jean Rouxel (IMN), University of Nantes, France
2 Department de Geologia y Mineralogia, Cudad Universitaria, Michoacan, Mexico
3 Laboratoie Miniralogie, MNHN, Paris, France
4 Laboratorie Gemmologie, University of Nantes, France

ABSTRACT
It is demonstrated here that the elementary building blocks of volcanic opals, mostly on examples from Mexico, are small silica grains, approximately 20 to 40 nm in diameter. They can group themselves in fibres, blades in lepîspheres or larger spheres for example.

The body colour in opal is generally due to inclusions, sometimes submicroscopic. In particular fire opal is coloured by nanometre-size fibres of an iron-containing compound.

W. Wm. Hanneman

Washington, USA

ABSTRACT
An innovative instrument, that is capable of quantifying the magnetic properties of gemstones, is described. The Hanneman Magnetic Index Meter uses the combination of a thin vertically suspending nylon thread and a rare earth magnet to record the angle of rotation of a rare earth magnet at which the gemstone breaks free of its magnetic attraction to the magnet. The scale of this instrument is calibrated in terms of the sine of the angle of rotation x 100.

Laboratory of Crystallography and Gemmology, Department of Chemistry,
Faculty of Science, Kasetsart University

ABSTRACT
The electronic and infrared spectra of 140 Möng Hsu rubies were analysed to find any differences between unheated and heat-treated specimens. There were no significant changes in electronic spectra in the region of 200-1100 nm that would indicate heat treatment. The red colour of ruby is due to the presence of Cr³+ giving rise to three d-d bands at 568, 424 and 300 nm. Several trace elements cause superposition on these bands. All spectra showed a fluorescence peak at 693nm. In infrared spectra of those rubies that had been heat treated, the peaks at 1900-2200 cm^-1 disappeared, and a series of sharp peaks was observed to replace the broad band at 3200-3500 cm^-1. 

Gemmological Institute of America

ABSTRACT
An illustrated description of the wood cell structure of opalised wood from the Rainbow Ridge opal mine, Virgin Valley, Nevada, USA that was visible when four specimens of this opal were examined with the microscope.

LOUISIANA OPAL

Grahame Brown

ALLGEM Services, Albany Creek, Queensland

ABSTRACT
Louisiana opal consists of a light to dark coloured sandstone that is cemented with opal, some of which displays the play-of-colour of precious opal. The deposit of opalised sandstone, which was discovered at Monks Hammock in central western Louisiana close to the Texas border, has not been mined commercially since the early 1990s. This rare variety of opal still can be purchased on the internet as slabs, cabochons, and spheres.

President, Gemmological Association of All Japan

ABSTRACT
It this paper techniques and observations suitable for discriminating Madagascan ruby and sapphire from similarly coloured rubies and sapphires from other world-wide localities are described. Methods for discriminating natural coloured from heat-treatment induced Madagascan ruby and sapphire also are presented. The usefulness of Laser Tomography to assist these discriminations is emphasized.

German Gemmological Association, Idar-Oberstein

ABSTRACT
This paper provides a description of various types of composite stones observed in a European gemmological laboratory. Besides classic, and commercially interesting types of doublets and triplets, some unusual specimens as well as assembled stones are included in this description.