3rd TAGRA

Sri Lanka's gem industry still relies largely on traditional exploration knowledge. Scientific studies have focused on floodplain and valley-fill alluvial deposits, and high-energy landforms such as plunge pools remain poorly examined. Plunge pools are scour basins at the base of waterfalls and may trap dense minerals such as gemstones; nevertheless, has not been tested in Sri Lankan gem-bearing rivers. This study assessed gemstone deposition potential in plunge pools of the upper Gala Oya River, Uva Province. The river rises on the Dorapoda mountain range (937 m) near Highland–Vijayan Complex boundary. Ten plunge pools were georeferenced, and waterfall height, pool depth, diameter, flow velocity and elevation were recorded. Two sand samples, one proximal and one distal to the waterfall, were collected from each pool, concentrated by panning and sieved. The 150 µm heavy mineral fraction was examined under a gemmological microscope, and grain counts were used to calculate the percentage of each variety. Relationships with pool parameters were tested using Pearson correlation, proximal–distal differences using paired t-tests, and spatial gem potential was mapped in ArcGIS using inverse distance weighting. Garnet was dominant (29.4–50.3%), followed by tourmaline (17.5–31.9%), yellow sapphire (2.8–9.6%) and zircon (0.6–4.5%). Yellow sapphire correlated strongly with pool depth (r = 0.89, p < 0.001) and moderately with diameter (r = 0.64, p = 0.045), while garnet declined with increasing elevation (r = −0.74, p = 0.015). Waterfall height and flow velocity showed no significant relationships, and no variety differed significantly between proximal and distal samples, so proximity to the waterfall is not a reliable exploration indicator. Despite the limited sample size and shallow sampling depth, the findings suggest that pool depth and size are practical morphological criteria for targeting yellow sapphire potential in plunge pools, while garnet potential is better explained by elevation. This supports low-impact, science-based gem exploration.