Analysis of the Refractive Properties of Selected Gemstones Found in Plateau State

Nigeria's Plateau State is among the richest gemstone deposits in sub-Saharan Africa, with commercially exploited minerals including topaz, sapphire, aquamarine, tourmaline, and zircon extracted from pegmatite formations in the Jos Plateau. Despite their economic importance, the optical and refractive properties of locally mined gemstones remain poorly characterized relative to internationally traded specimens. This study measures the refractive index, birefringence, and optical dispersion of twelve gem specimens of five mineral types collected from Jos South and Barkin Ladi Local Government Areas in Plateau State, North Central Nigeria. Measurements were conducted using a gem-quality Abbe refractometer and calibrated immersion liquids, following standard gemmological procedures. Polarizing microscopy was applied to assess birefringence and crystal system classification. Results indicate refractive index values consistent with published international ranges for most specimens, with topaz samples yielding values of 1.618 to 1.627, consistent with literature. However, two zircon specimens exhibited anomalously low birefringence compared to standard values, suggesting metamictization due to radiation damage from internal uranium and thorium impurities. The study identifies optical markers that could be used to distinguish Nigerian-origin gemstones from synthetic or foreign equivalents in gemological authentication, offering commercial value to the Nigerian gem export trade. Recommendations are made for integrating optical characterization into the quality certification standards of Nigeria's mining regulatory framework. Keywords: refractive index, gemstones, Plateau State, optical properties, gemmology