Opals are widespread within Miocene volcanic sequences in the North Shoa province of Central Ethiopia. The opal occurs as cavity fillings in a 5 m thick seam of glassy rhyolitic ignimbrite that is sandwiched between basaltic lava flows. The opals occur over a large area (>25 km2). X-ray diffraction analyses show that they are CT-type. The opals contain lower concentrations of trace elements (up to 100 times) than the host rhyolite. Ratios of most trace elements are, however, similar in both the opals and rhyolites. The opals have high δ18O values (28.4–33.8‰) that imply a low temperature of formation, between 20.55 and 25.74 °C. We therefore propose that the opals precipitated from meteoric waters that had percolated through and interacted with the host rhyolite. Field evidence indicates that this weathering and alteration occurred immediately after emplacement of the rhyolites, but prior to the extrusion of the overlying basalt flows.