Abstract Two distinct mineralogic types of opal are found in the various members of the Neogene Ogallala Formation of western Kansas, where the opal occurs in abundant diatom tests, has replaced wood, has partly replaced siliceous roots and seeds, cements feldspathic sandstone, occurs in bentonite beds, and is present as discontinuous massive beds below a surface of late Pliocene and early Pleistocene weathering. Four modes of origin are recognized: (1) biogenetic precipitation of opal as diatom tests; (2) addition of opal to silica-containing plants; (3) desilication of volcanic glass and precipitation of the silica in underlying rocks as opaline cement and replacement of wood; and (4) desilication in conjunction with calcium carbonate calichification below a late Plioceneearly Pleistocene weathering surface and precipitation of the silica as discontinuous opaline beds in the upper part of the Ogallala Formation.