The color origin of gem diaspore: Correlation to corundum

Abstract

Color-change diaspore, known commercially as Zultanite, is sought by designers and consumers for its special optical characteristics, namely its color and color change. Understanding the color origin of gem-grade diaspore could provide a scientific basis to guide its gemological testing, cutting, and valuation. This study uses ultraviolet- visible (UV-Vis) spectra and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to examine the color origin of color-change diaspore and to compare it with corundum. As Raman spectra vibration intensities are closely related to crystal direction for diaspore, crystal orientation was determined through Raman spectroscopy. The color correlation between color-change diaspore and corundum confirmed the identity of each chromophore. In addition, the effectiveness of different chromophores such as Cr3+, Fe3+, Fe2+-Ti4+ pairs, and V3+ between gem-quality diaspore and corundum is compared quantitatively.