Unstable Colouration of Padparadscha-like Sapphires

Abstract

After the October 2016 discovery of a new gem deposit at Bemainty near Ambatondrazaka, Madagascar, a number of sapphires with padparadscha-like colour entered the trade. However, most of these stones were found to have unstable colour, which changes from pinkish orange to more-or-less pure pink after a few weeks in daylight. In this study, the authors investigate the colour stability of padparadscha-type sapphires of metamorphic origin-mainly those originating from Madagascar (Ambatondrazaka and Ilakaka) and Sri Lanka. The 48 samples could be separated into three groups after colour-stability testing: sapphires that did not show a noticeably different appearance (case A); sapphires with a slight-to-moderate colour difference within the padparadscha range (case B); and fancy-colour sapphires showing a distinct change in appearance that fell outside of the padparadscha range (case C). The last situation was especially common for the stones from Ambatondrazaka, thus revealing that careful colour-stability testing is mandatory for proper gemmological identification of any sapphire showing a yellow to orange colour component.

The Covariation of Color and Orange Fluorescence Instabilities in Yellow Sapphires

“…This color instability is called photochromism [1]. Photochromic effects are known in a variety of gemstones, including diamond [2,3], sapphire [4,5], hackmanite [6], scapolite [7], tugtupite [8], and zircon [9,10], among others. For high-value colored gemstones such as sapphire especially, subtle color changes mostly degrade the level during color classification, thus lessening their value.…”

“…Sapphire, as one of the highest-value colored gemstones exhibiting the photochromic effect, has been extensively studied. Most studies since the 1940s have focused on photographic records and descriptions of color instability [4,5,[11][12][13][14][15]. For instance, Pough and Rogers (1947) first reported the photochromism of natural sapphire [11].…”

“…Furthermore, Williams and Williams (2016) and Zhao et al (2018) indicated that natural, synthetic, and heat-treated yellow sapphires can display the photochromic effect, especially those from Sri Lanka and Madagascar [13,14]. Later, this phenomenon was observed not only in yellow sapphires, but also in Papalacha-like and light to colorless sapphires [5,15]. From these considerations, one may infer that the photochromic effect is related to the intrinsic characteristics of corundum crystals, which occur in a variety of colors and natural, synthetic, and heat-treated samples.…”