This study was undertaken to investigate the feasibility of surface geochemical techniques applied to exploration for coloured gemstones in the Mangare area, SE Kenya. At the study site, the Rockland (lately John Saul) Ruby Mine, gem corundum mineralization is found in a pegmatite reef lining the boundaries of a major ultramafic body with gneisses. Overburden, 1 to 3 m thick, is represented by secondary limestone, gravels and soils. The latter were subjected to bulk sampling and in situ selective leach by the Method of Diffusion Extraction (MDI). Anomalous Ni, Co and Cr individual and product values were returned from the bulk soil and MDI determinations over the concealed ultramafics (UM). The MDI anomaly gave greater definition of the UM body. Additional anomalies were identified in the calcareous soils which appear to have a superior ability to accumulate trace elements susceptible to MDI extraction. However, this landscape effect can be eliminated by differential data treatment. In addition, the planar distribution of an artificial factor, the Local Empiric, is shown to delineate the pegmatite zone, regardless of soil type. It is concluded that the most cost-effective approach would involve bulk sampling over a loose grid to define ultramafic belts, followed by MDI or other specific techniques to accurately delineate the individual bodies and their margins.