The Color Origin of Gem Diaspore: Correlation to Corundum

Abstract

position in the gem market due to its rarity, striking pleochroism, and color-change phenomenon (figure 1). The material’s value depends on these factors. A clear understanding of color origin offers considerable benefits for gemological testing, cutting, and even valuation of gem diaspore. By replacing the major elements in definite structural units through isomorphous substitution, trace elements play an important role in the color of gemstones. The AlO6 octahedra is a significant structural unit that produces color when different trace elements substitute for Al. For example, Cr3+ substitutes for Al3+ in the AlO6 octahedra in jadeite and spinel, causing green and red color (Lu, 2012; Malsy, 2012), while the substitution of Fe3+ for Al3+ in sapphire produces yellow color (Emmett et al., 2003). Diaspore and corundum have a similar chemical composition and crystal structure (see figure 2). Diaspore, with the chemical formula AlO(OH), belongs to the orthorhombic space group 2/m 2/m 2/m (Hill, 1979); corundum, with the chemical formula Al2O3, belongs to the trigonal space group 3̅ 2/m (Lewis et al., 1982). The crystal structure of diaspore consists of AlO4(OH)2 octahedra, whereas the corundum crystal structure consists of AlO6 octahedra (Hill, 1979; Lewis et al., 1982). Both types of crystals are composed solely of octahedral units. In addition, the diaspore structure is able to convert to corundum structure through dehydration (Iwai et al., 1973). Due to their closely related crystallographic structure and chemical composition,