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Natural Gamma Radioactivity and Exploration for Precious Opal in Australia
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Opal carvings by Daniela L’Abbate. Photography Jenni Brammall (Left 10.8,
centre 38 & right 58 carats). Courtesy of OTRAD Pty Ltd, S. Tranter-Brown and D. Sanders.

Brian R. Senior¹ and Lewis T. Chadderton^2
1 Senior & Associates, Bungendore NSW, Australia.
² Atomic and Molecular Physics Laboratories, Australian National University, Canberra, ACT.

ABSTRACT
A microscopic superlattice arrangement of silica microspherulites, 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 opencut mine walls demonstrates natural radioactivity surrounding opal deposits, 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.