A geode from Sarusas, Namibia, consisting of zones of quartz and agate has δ18O values (20,4‰-27,0‰) which indicate that it crystallized at a temperature of <200°C and possibly < 100°C. Differences between δ18O of quartz and agate in the geode may be explained if the quartz crystallized from H2 O vapour and the agate crystallized from H2 O liquid in equilibrium at about 102°C. This model can explain how quartz and agate bands are formed in such geodes.