Hard rock terrains, defined here as crystalline igneous-metamorphic aquifers in which groundwater is stored mainly in weathered mantles and fracture networks, are especially prone to borehole-scale fluoride variability that complicates rural drinking-water security. This mini-review synthesizes recent evidence on geogenic fluoride occurrence, exposure pathways, and defluoridation strategies relevant to such settings. Elevated fluoride is most consistently associated with weathering of F-bearing