The Study On Gemological And Mineralogical Characteristics Of Nigerian Emerald

Emerald is among one of the most popular gemstones. Apart from traditional deposits such as Columbia and Brazil, emeralds from Africa are becoming more and more important in the world’s emerald market. As one of the typical emerald deposits in Africa, Nigerian deposits are associated with Pan-African Granites. So the research of them might help in understanding the African emerald deposits as a whole and make contribution to the further discovery of new emerald deposits.Emerald deposits are in the Plateau and Kaduna States, central Nigeria. Emeralds occur in pegmatite veins or gravities of granites. They appear pale green to blue-green and are characterized by large size and good form. In this research, gemological and mineralogical characteristics of Nigeria emerald are described and a model of emerald formation is presented.Nigerian emeralds show relatively low refractive indices and specific gravities due to their low content of Na. Other chemical character is low Mg and high Fe. The absorption spectra of Nigerian emeralds illustrates that besides Cr, V, their color also attribute to Fe, which also gives rise to Nigerian emerald a hint of blue color. Relatively high Fe can distinguish Nigerian emerald from the other deposits.Internal features were observed and photographed by using gemological microscope with Nikon camera and some were determined by using confocal-Raman spectroscope. The main solid conclusions are albite, fluorite, ilmenite, monazite, quartz, tourmaline, beryl, rutile, spinel, Fe-rich biotite and multi-phased black aggregate(which contains ilmenite + rutile +/- cassiterite +/- niobite +/- Fe oxides/hydroxide). The last two are Nigerian exclusive inclusions. The mineral association of albite + fluorite + monazite + rutile is also locality-specific. Fluid inclusions are determined by FTIR spectrometer and Raman Spectrometer, which indicates two-phased fluid inclusions can be divided into two types: a. volatile CO2 + liquid H2O; b. volatile CH4 + liquid H2 O. Besides, there’re also three-phased fluid inclusions and multi-phased fluid inclusions.Nigerian emerald occurs in alkaline granites. Emeralds are probably formed during the late magmatic and early subsolidus hydrothermal stage. The element Cr and V may come from basement rocks and Be comes from fluid that derives from magma.