Colour Enhancement of Pink Tourmaline from Nigeria by Electron-Beam and Gamma Irradiation

Abstract

Saturated pink-to-red tourmaline is a desirable gem material and therefore pale stones are commonly enhanced by gamma irradiation to intensify the red colour. For this study, 20 tourmalines from Nigeria ranging from near-colourless to pale pink, pink and pale orangey pink were subjected to electron-beam and gamma irradiation. Each sample was cut into two pieces for irradiation by each treatment method at three dose levels (i.e. 400, 800 and 1200 kilograys). During treatment, the samples turned pink to intense pink and orangey pink to pinkish orange or yellowish orange, with more intense colouration produced by electron-beam irradiation. The UV-Vis spectra of the irradiated tourmalines showed a maximum absorption feature centred at around 510–520 nm (due to Mn3+) and a minor band at about 395 nm also attributed to Mn (probably Mn3+), which caused the pink colouration. Yellow colouration seen in some samples after irradiation was probably contributed by the O– hole centre. Features associated with both the O– hole centre and H0 electron centre were present in the EPR spectra of the irradiated samples. © 2021 Gem-A (The Gemmological Association of Great Britain)