Demantoid from Kerman Province, South-east Iran: A Mineralogical and Gemmological Overview

Andradite, ideally Ca₃Fe³⁺₂Si₃O₁₂, is the Ca-Fe end member of the ugrandite garnet subgroup and occurs in three colour-related varieties: black melanite, yellow to brown topazo-lite and yellowish green to green demantoid (Figure 1; O’Donoghue 2006). Demantoid usually contains traces of Cr, which is the main cause of the green colour, although stones from some deposits lack this element.

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In 1853, Finnish mineralogist Nils Gustaf Nordenskiöld first described demantoid as a new variety of andradite from Nizhniy Tagil in the Central Ural Mountains of Russia (Eichmann 1870). Due to its exceptional brilliance and high dispersion, Nordenskiöld named the material demantoid (‘diamond-like’). With a dispersion higher than that of diamond and its vivid green colour, it quickly became au courant at the Russian royal court as a treasured and expensive gem material.

For decades, demantoid was found almost exclusively in Russia (Phillips & Talantsev 1996). More recently, several other sources of gem-quality demantoid have been reported, such as Canada, Iran, Italy¹, Madagascar, Pakistan and Namibia (Lind et al. 1998; Quinn & Laurs 2005; Du Toit et al. 2006; Karampelas et al. 2007; Adamo et al. 2009; Pezzotta et al. 2011). Most reports on demantoid have focused on its occurrence, crystal chemistry and gemmological properties (Gill 1978; Misiorowski & Hays 1993; Krzemnicki 1999; Milisenda et al. 2001; Pavese et al. 2001; Quinn & Laurs 2005; Adamo et al. 2015).

Gem-quality demantoid is commonly found in two very different geological settings (Adamo et al. 2011): serpentinite (e.g. Kerman Province, Iran; Val Malenco, Italy; Kaghan Valley, Pakistan; and Ural Mountains, Russia) and skarn (e.g. Antetezambato, Madagascar; and Erongo, Namibia). Most demantoid from serpentinite contains very fine and often curved needles, predominantly consisting of chrysotile, Mg₃Si₂O₅(OH)₄, a member of the kaolinite-serpentine group. These inclusions are characteristic of demantoid from Iran, Italy, Pakistan and Russia (Palke & Pardieu 2014). They do not, however, occur in skarn-related demantoid from Madagascar or Namibia. In addition, the chrysotile fibres in demantoid from serpentinites often radiate from a central chromite inclusion (CrFe₂O₄), forming what is commonly referred to as...