Purple Gem Spinel from Vietnam and Afghanistan: Comparison of Trace Element Chemistry, Cause of Color, and Inclusions

Abstract

GEMS & GEMOLOGY FALL 2021 Gem-quality spinel occurs in a variety of colors as a result of a diverse array of possible chromophores and chromophore combinations (Schmetzer et al., 1989; Andreozzi et al., 2019): combinations of Fe2+ and Fe3+ (light blue/violet to green to black with increasing Fe concentration; Hålenius et al., 2002); Co2+ (blue; Shigley and Stockton, 1984; D’Ippolito et al., 2015); Cr3+ (red); and V3+ (orange; Andreozzi et al., 2019). Vivid spinels colored by cobalt (blue) and chromium (red) are particularly sought after in the gem trade. Purple spinel (figure 1) is generally colored by Fe or by a combination of Cr, V, and subordinate Fe (Andreozzi et al., 2019). The current study compares the chemical composition and color of two purple spinel samples, from Vietnam and Afghanistan (figure 2), to each other and in the PURPLE GEM SPINEL FROM VIETNAM AND AFGHANISTAN: COMPARISON OF TRACE ELEMENT CHEMISTRY, CAUSE OF COLOR, AND INCLUSIONS