The Twin Knobs #1 diatreme (TK1), delineated in 1981, is located 3.8 km southeast of Murfreesboro, Pike County, Arkansas and is within the northeast-trending group of pipes, plugs, and dikes that comprise the Murfreesboro district. TK1 is an elongate, bilobate pipe, oriented north-south, and approximately 0.05 km 2 in surface area. The diatreme penetrated the Lower Cretaceous Trinity Formation, whereas the Upper Cretaceous Tokio Formation overlies portions of the pipe. These relationships suggest an emplacement age of early Late Cretaceous, which is contemporaneous with isotopic ages obtained at the Prairie Creek lamproite complex (97 to 106 Ma; Gogineni and others, 1978). Rock types at TK1 include magmatic lamproites, sandy tuffs and breccias, and epiclastics. The magmatic breccias are porphyritic, with euhedral olivine, plus pyroxene and phlogopite phenocrysts set in a cryptocrystalline or glassy groundmass that also contains priderite, perovskite, and chromite. The sandy tuffs and epiclastics contain up to 30 percent quartz grains, in addition to lamproite and sedimentary rock fragments, set in a matrix of calcite, sericite, or fine-grained to cryptocrystalline glass. The average diamond grade from bulk sampling of the phases is 0.17 carats per 100 metric tons. Diamond crystal habits include octahedra, tetrahexahedra, macles, and fragments. Megacrysts of picroilmenite, chrome spinel, clinopyroxene, orthopyroxene, garnet, and nodules of eclogite, peridotite, Iherzolite, websterite, as well as crustal xenoliths, were recovered during sampling. Whole-rock and trace element geochemistry strongly suggests that Twin Knobs #1 has more affinities to olivine lamproite than to kimberlite.