Volume 23, Number 4, October - December 2007

Magnetic Susceptibility for Gemstone Discrimination Opal Exploration Using Natural Gamma Radioactivity Colour Analysis with LED Lightsources

Dr D. B. Hoover FGA, FGAA (Hon.)¹, B Williams^2
1 Hoover Associates, Springfield, Missouri, USA
² Bear Essentiuals, Jefferson City, Missouri, USA

ABSTRACT
Following a review of the physics of magnetism and magnetic susceptibility, the authors present a simple method for the quantitative measurement of magnetic susceptibility on cut gemstones. The method requires a scale such as commonly used for measurement of weights of cut gemstones, a few inexpensive magnets, and a simple device to slowly place and then lift the magnet from the gemstone's surface so as to measure the force of attraction

The technique is applied to a variety of gemstones, but is focused on corundum and peridot. The authors believe that magnetic susceptibility may be able to distinguish between the magmatic or metamorphic genesis of corunda on the basis of their iron content as indicated by their magnetic susceptibility. Similarly, peridot may be categorised from its iron content based on magnetic susceptibility, and is easily distinguished from sinhalite.

Opal Exploration Using Natural Gamma Radioactivity Brian R. Senior1, Lewis T. Chadderton2 1 Senior and Associates, Bungendore, NSW, Australia 2 Atomic and Molecular Physics Laboratories, Australian National University, Canberra, ACT ABSTRACT A microscopic superlattice arrangement of silica rnicrospherulites, each with a small central nano-nucleus, followed by Bragg diffraction of white light, gives precious opal its pseudochromatic 'fire'. Pre-sedimentation heterogeneous microspherulite growth within siliceous groundwater takes place on a central radioactive catalyst core. Neutron activation and secondary ion spectroscopy (SIMS) of precious opal reveals thorium, uranium, lead, and anomalous concentrations of daughter nuclides from natural uranium fission. Gamma-ray radioactive logging of drill holes and open-cut mine walls demonstrates natural radioactivity surrounding opal deposit's increasing in intensity towards the central precious opal zone. Case histories of new gemstone discoveries are described, and the high intrinsic value of this unique technique confirmed.

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LED Lightsources and gemmological
instruments that analyse colour

Trevor Linton
Linton Enterprises, Wellington Point, Queensland, Australia

ABSTRACT
The practical use of 'white' Light Emitting Diodes (LEDs) with gemmological instruments that analyse various components of a gemstone's colour are a combination that requires knowledge of the precise nature of spectra emitted by LED light sources. A critical evaluation suggests that when using a dichroscope in association with LED light sources, this combination will produce anomalous dichroic or pleochroic colours from many coloured gemstones.

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