Fossil driftwood from Sarmatian (Late Miocene) gravel deposits near Yakuplu, Turkey, ranges between lignitic and opaline. Opalization occurred in various stages of decay. The stage of decay of the wood and the eventual admixture of detritic clay influenced the crystallinity and the dehydration characteristics of the opal. Opals influenced by the host rock seem to have montmorillonitic suprastructures. Opalization was probably the result of the weathering of volcanic ash layers in the overlying strata.