The Heat Treatment of Pink Zoisite

Abstract

Natural pink zoisites owe their color to a high concentration of manganese paired with low concentrations of other coloring elements such as vanadium or titanium. Upon conventional heating, such stones typically suffer from the reduction of Mn3+ to the colorless Mn2+ species alongside the destruction of the brownish yellow color that is related to titanium. We have processed manganese containing zoisites under the high pressure of pure oxygen which allowed the manganese to remain oxidized, while the brownish yellow color component was still successfully removed. Depending on the vanadium level, the treated gems show a pink to purplish pink color. Detection of this treatment is not easy as the temperature is too low to result in a change in internal features, but a combination of UV-Vis-NIR spectroscopy and trace element chemistry provided by LA-ICP-MS give evidence of such treatment.

Editorial for the Special Issue “Gem Characterisation”

“…Treatments to improve the colour, appearance and/or durability of gems have been used for several years; over recent decades, these have increased and become more sophisticated [10][11][12][13]. In the present issue, four studies on the treatment of gems are published [14][15][16][17]. Zhou et al [14] present the results of the method they developed for the hightemperature copper diffusion process for the surface recolouring of faceted labradorites.…”

“…In parallel, they describe their gemmological and spectroscopic characteristics. The heat treatment of pink zoisite is presented by Schwarzinger [15]. This paper describes the heat treatment of zoisites under pure oxygen which allowed the manganese to remain oxidized, while the brownish yellow colour component was still successfully removed.…”