This study presents the geology and mineralogy of the agate nodules that occur in the Pliocene polymictic conglomerates, northwest of Dereyalak village, Eskisehir, northwest Turkey, and their relationship to sepiolite nodules. The host-rock of the agates is a layer 5-25 m thick with an outcrop length of more than 1.5 km. This host rock consists of well-rounded pebbles, mainly composed of opal, ultramafics, sandstones, limestones and volcanics. The conglomerates unconformable overlie an Upper Cretaceous ophiolite complex. This tectonic contact, which trends E-W, is accompanied by another NE-SW-trending major fault. Most of the agate nodules are present along these fault zones.