Study of the Optical Emission Spectrum of Selected Nigerian Artisanal Gemstones Using a Diffraction Grating Spectrometer

Optical emission and absorption spectroscopy are standard mineralogical identification techniques that exploit the unique spectral fingerprints arising from electronic transitions in mineral crystal structures. This study characterizes the optical emission spectra of eight artisanal gemstone specimens collected from mining sites in Ekiti and Ondo States, South West Nigeria, using a diffraction grating spectrometer operating over the visible wavelength range of 380 nm to 750 nm. Samples of emerald, tourmaline, amethyst, garnet, beryl, topaz, epidote, and blue sapphire were examined under illumination by a halogen broadband source and a focused UV lamp to observe both absorption features and luminescence emission. Spectra were recorded and analyzed to identify characteristic absorption bands associated with transition metal chromophores including chromium, iron, manganese, and titanium. Emerald samples showed characteristic chromium absorption doublet bands at 600 nm and 630 nm consistent with Colombian and Zambian type standards. Garnet specimens displayed iron-related absorption at 510 nm and 565 nm. The study investigates whether spectral profiles can reliably distinguish Nigerian-origin gemstones from those mined elsewhere based on trace element impurity signatures. Multivariate analysis of spectral data using principal component analysis reveals distinct clustering of specimens by mineral type. The research contributes spectral reference data to support gemological authentication efforts by the Nigerian Mining Cadastre Office. Keywords: optical emission spectroscopy, gemstones, diffraction grating, Ekiti State, mineral identification