Introduction

VIII-1.

Terrestrial background radiation in the host rock of a gemstone deposit can alter the colour of the gemstone material if the radiation dose is high enough and the ambient temperature low enough. For example, tourmalines from gemstone pegmatite become pink or red from exposure to high energy (1.46 MeV) gamma rays from ⁴⁰K over periods of millions of years. The natural blue colour of some topaz is thought to be produced by natural irradiation, as is the deep blue colour of Maxixe beryl. The surface coloration of yellow and yellow-green diamonds has also been attributed to natural radiation [VIII–1].

VIII-2.

The natural processes of gemstone coloration induced by background radiation can be accelerated by artificial means. A number of irradiation technologies are used around the world to enhance the colour and beauty of many different gemstones. This irradiation can produce gemstones in colours that are not found or are extremely rare in nature, which can significantly increase their commercial value. For example, the value of topaz can be increased up to ten times by using colour enhancement through irradiation [VIII–2]. There is also evidence of increased hardness of irradiated gemstones, another factor that adds to their commercial value [VIII–3, VIII–4].

VIII-3.

A wide variety of gemstones are enhanced in colour by irradiation [VIII–5 to VIII–8]. The most common colour changes that can be induced are listed in Table VIII–1.

VIII-4.

Three different processes are routinely used in the irradiation of gemstones to enhance their appearance and colour. These processes involve irradiation by neutrons (in a research reactor), by electron beams (using a linear accelerator) or by gamma emitting radionuclides [VIII–8]. In some instances, more than one process (e.g. irradiation by neutrons followed by electron beam irradiation) may be applied.