Mineralogical Study of Gemstone Occurrences in Oyo State, South West Nigeria

Oyo State in South West Nigeria, underlain by Precambrian basement complex rocks including migmatite-gneiss complexes and younger granites, hosts a variety of gemstone occurrences including tourmaline, beryl, garnet, and quartz varieties that have attracted artisanal prospecting but remain poorly characterised in the scientific literature. This study undertakes a mineralogical investigation of gemstone occurrences at selected pegmatite and alluvial sites in Oyo State, with the objective of characterising gem mineral species, assessing quality attributes, and evaluating economic potential. Fieldwork involved systematic outcrop mapping and sampling of pegmatite bodies, with particular attention to gem mineral occurrence patterns relative to pegmatite zonation. Alluvial heavy mineral samples were collected from active stream gravels and panned to recover gem concentrates. Identified specimens were characterised by X-ray diffraction, polarised light microscopy, and Raman spectroscopy to confirm mineral identity. Physical properties including colour, clarity, hardness, and specific gravity were measured to assign gem quality grades following gemological classification standards. The study evaluates the composition and size characteristics of tourmaline and beryl occurrences against published data from commercially productive pegmatite provinces. Recommendations are made regarding the feasibility of formalising artisanal operations under the government's gemstone licensing framework. Keywords: gemstones, pegmatite, mineralogy, Oyo State, tourmaline.