The world＇s main sources of non-alluvial diamonds are found in ultrapotassic kimberlite(1) or lamproite(2) diatremes (pipes filled with explosive volcanic debris), most of which have Phanerozoic ages and are located in stable Precambrian cratons, Diamond exploration has therefore tended to focus on such deposits. Microdiamonds are known to occur in metamorphic rocks such as gneiss(3) and eclogite(4) that have equilibrated deep in the mantle and were then tectonically transported to the surface, but such deposits are thought to have little commercial potential. Here we report a new type of diamond occurrence from the Dachine region in French Guiana for which the host rock is volcaniclastic komatiite-an unusual type of volcanic rock whose composition and origin are quite unlike those of kimberlite and lamproite. These komatiites form part of a Proterozoic island-are sequence, a tectonic setting distinct from that of all other currently exploited diamond deposits. The discovery of diamonds in volcaniclastic komatiite has implications not only for diamond exploration, but also provides strong evidence that these komatiite magmas originated at depths of 250 km or greater within the Earth.