engleski

Gemmological properties of gamma irradiated brazilian topaz

Natural topaz occurs in a variety of colours. With the exception of some pink and red stones where trace quantities of chromium act as a chromophore, the colour in topaz is due to colour centres. This makes topaz suitable for colour change through irradiation and heating. Most natural topaz is colourless or very pale blue. The darker blue colour, so commonly seen today is produced by irradiation, usually followed by heat-annealing. Different irradiation processes are used involving neutrons, electrons, beta and gamma rays. Today, gamma irradiation is mostly used as a "pretreating" or screening process in order to identify material that is likely to be susceptible to treatment with higher energy techniques. All irradiation techniques invlove the creation, and sometimes partial destruction, of colour centres. However, none of the employed techniques produce "characteristic" colours that can with certainty be ascribed to a particular treatment. At present, there is no definite gemmological method for differentiating among stones treated by the various techniques or for distinguishing between treated and untreated stones. In the present work we show that the colour of natural brazilian topaz can be changed by gamma irradiation. Crystals and "pebbles" (from placer deposits) were irradiated by a Co-60 source located at the Rudjer Boskovic Institute. The samples received an initial gamma dose of 6 MGy, after which irradiation was continued up to a total dose of 45 MGy. The irradiated material was subsequently heated on a hot plate in order to remove the brown colouration. A transparent light blue colour remained, the hue and intensity depending on the gamma dose. The changing of naturally off-coloured or colourless topaz by gamma irradiation has the advantage over irradiation by neutrons in a reactor in that gamma irradiation causes no nuclear activation of the topaz. The resulting colour of the irradiated topaz material shows no zonation or patchiness. Both cut stones and natural crystals were subject to the standard set of gemmological determination techniques, the results of which will be discussed.

topaz; gamma irradiation; Brazil

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