Abstract: This study presents the geology and mineralogy of the opal nodules which occur in the Pliocene carbonatedconglomerates, NW of the Yenisofça village (Eskişehir, NW Turkey). The host-rock of opals consist of well-roundedpebbles, mainly opal, ultramafics (mainly serpentinites), and micritic limestones. The mainly white, transparent andgrey colored opal nodules range from 2 to 30 cm in diameter The X-ray diffraction (XRD) analyses revealed opalCT, quartz, dolomite and sepiolite as the main mineral components of opals. Scanning electron microscope (SEM)images show that sphere and fibrous textures are found in opal nodules. The comparison of fibrous textures observedin the opal nodules with those observed in sepiolite nodules based on SEM images showed that there is a similaritybetween them. The major element composition of the opal by SEM and whole rock geochemical analyses reveal thatthe concentration of the MgO is higher than the other oxides in the opals. Field studies, geochemical analyses, andmineralogical data suggest that the Yenisofça opals could have formed by sepiolite replacement by low temperaturesilica-rich hydrothermal solutions that circulated along the fractures systems. This result is similar and coherentwith the properties and formation of opal and cryptocrystalline quartz occurrences outcropping in the Eskişehir andKütahya regions.