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We have selected samples from C.I.G. 's comprehensive gem study collection which are difficult to identify. Most pictures were taken with an immersion-scope (methylene iodide or benzyl benzoate); under a common "dry" microscope with dark-field illumination many features shown here may not be visible. A detailed booklet with computer diskette containing over 100 micro-photographs is available from Gemlab Book Services . © 1996 J. W. Kuehn

Description of inclusion features have been provided by Peter G. Read

802Gilson Opal

As there's a significant difference between the chemical composition of Gilson opals and natural opals, it's more correct to regard them as simulants rather than synthetic opals. Under U-V (particularly L.W.), the Gilson opals show a distinctive chalky bluish-white body glow and a much stronger surface fluorescence of the same colour.

When viewed under highintensity fibre-optic illumination, a "chicken wire" structural effect can be seen. Under normal illumination, the typical "lizard skin" structure present in both black and white Gilson opals is also visible.

803Gilson Black Opal

804Slocum Stone, opal glass imitation

805Opal, natural, Australia

806Black Opal, natural, Australia

807Gilson Opal Doublet

809Plastic Opal Simulant