Large Faceted Gahnospinel
Gahnospinel is a rare dark greenish blue gemstone that belongs to the spinel group. Its properties result from being part of a solid solution series with the end members gahnite (ZnAl2O4) and spinel (MgAl2O4). Because of the high amount of zinc in the mineral, the refractive index (RI) and specific gravity (SG) shift from lower values for the spinel end member to higher values as the amount of Zn substituting for Mg increases, which may lead to confusion when trying to identify it. Spinel has an SG of 3.06 and an RI of 1.718, while gahnite has an SG of 4.55 and an RI of 1.800. As a combination of the two, gahnospinel’s SG can fall anywhere in between.
An 11.34 ct transparent faceted oval mixed cut (see above) was recently submitted to the Carlsbad lab for an identification report. Despite the stone’s large size, the only inclusions were tiny crystals. To date, this is the largest gahnospinel seen at any GIA laboratory worldwide. The previous stones submitted were 1.95 ct and below. Standard gemological testing revealed an RI of 1.754 and an SG of 4.10. These results are not typical for spinel. Inductively coupled plasma–mass spectrometry (ICP-MS) data revealed a very high zinc content and the stoichiometry of the sample was calculated to be the following: (Mg0.485Zn0.385Fe2+0.043)0.913(Al2.049Si0.004)2.053O4. The atomic mass of zinc is 65.38, far greater than the 24.31 atomic mass of Mg. As the amount of zinc substitution for Mg in gahnospinel increases, it tends to have a higher RI and specific gravity than spinel. This variety of spinel may bring identification challenges without advanced testing techniques to identify the presence of Zn.
