Nairobi, April 20 -- Across Vihiga County in western Kenya, massive granite boulders dot the landscape, spilling across hillsides, farmlands and open fields. They are silent markers of a vast mineral resource long seen as both a blessing and a missed economic opportunity.

This is because beneath the rugged, and sometimes mossy, surfaces of the boulders, or miamba as they are popularly known locally, lie reddish and greyish stones, which have for years been quarried in small quantities by artisanal miners and sold cheaply in raw form.

These blocks are then cut, polished and processed into high-value construction materials such as floor tiles, countertops, wall cladding, gravestones, terrazzo chips and ballast-products which fetch far hig...