gems, beryl, emerald, Nigeria

Mineralogical and gemmological characteristics of emeralds and green beryls from the central Nigerian Kaduna and Plateau States are presented. A genetic model has been established for the occurrence of these gemstones within greisen associations of Mesozoic alkali granite ring complexes. Gemmologists should be able to distinguish Nigerian emeralds from those of other origins based on their unique inclusions and chemical characteristics. In Nigerian specimens containing solid inclusions, the specific mineral assemblage of albite + fluorides (fluorite, boldyrevite, ralstonite) + F-silicate + Fe-rich mica + ilmenite + monazite is diagnostic of the locality. Growth structures and fluid inclusions are extremely common; the latter display a wide range of features, including Colombian-type three-phase inclusions. For Nigerian emeralds exhibiting an inclusion pattern similar to Colombian material, the most reliable distinction is via absorption spectroscopy. Nigerian emeralds display ‘mixed spectra’ with absorption peaks due to Cr³⁺, Fe²⁺, Fe³⁺, and Fe²⁺/Fe³⁺, whereas Colombian emeralds typically show spectra largely devoid of iron-related features.