The sources of rare diamonds reported in northwestern British Columbia, southwestern Yukon, and parts of Alaska are enigmatic. We carried out a heavy-mineral survey of 17 streams draining bedrock in the Atlin–Nakina region in northwestern British Columbia to determine if high-pressure igneous and metamorphic rocks exhumed in the area could be potential sources for the diamond. Heavy-mineral fractions returned flakes of gold but no diamond. The ferro-magnetic fractions were examined optically and by electron microprobe analysis of key indicator minerals, namely olivine, orthopyroxene, clinopyroxene, garnet, spinel, and titanite. Detritus from a horizon of coarse pebble conglomerate in the Jurassic Laberge Group south of Atlin is the most likely source of the anomalous diamond. Garnets and pyroxenes in the latter sedimentary unit were derived by rapid erosion of peridotite and eclogite massif bodies exhumed from depths approaching the diamond stability field during collision and Pliensbachian uplift in the northern Cordillera. Evidence is shown for glacial transport of detritus from both the Laberge Group sediments and Neogene volcanics, the latter of which evidently covered a much wider area before the last glaciation.