Gem News International Contributing Editors Emmanuel Fritsch, University of Nantes, CNRS, Team 6502, Institut des Matériaux Jean Rouxel (IMN), Nantes, France (
[email protected]) Gagan Choudhary, Gem Testing Laboratory, Jaipur, India (
[email protected]) Christopher M. Breeding, GIA, Carlsbad (
[email protected]) I ) I ------------------------------------------------------------ 1_______________________________________________________________ _ COLORED STONES AND ORGANIC MATERIALS Blue gahnite from Nigeria. In recent years, Nigeria has gained considerable attention in the gem trade for the dis- covery of sapphires reportedly from the Mambilla Plateau in Taraba State. In addition to sapphires, spinels have been sporadically mined in Jemaa and recently in Kagoro, both in Kaduna State (figure 1). The spinel group of minerals, with a general chemical formula AB O , has a total of 22 2 4 different species. Four of these species have Al predomi- nantly occupying the octahedral site: spinel (MgAl O ), 2 4 galaxite (MnAl O ), hercynite (FeAl O ), and gahnite 2 4 2 4 (ZnAl O ). The spinels from Kagoro have been identified 2 4 as gahnite, the zinc end member. Gahnite is one of the rarer members of the spinel group of minerals, typically found at zinc deposits. Most crystals are very small, in- Figure 1. Three single octahedra of natural gahnite, ap- cluded, and translucent to opaque. Faceted gems are very proximately 1.5 ct each, from Kaduna State, Nigeria. collectible due to their rarity and color. Photo by Robison McMurtry. The geology of the Nigerian spinels has been previously studied (R. Jacobson and J.S. Webb, “The pegmatites of cen- tral Nigeria,” Geological Survey of Nigeria Bulletin, No.