Several pieces of fossil wood have been analyzed with X-ray diffraction and were grouped on the basis of mineralogical composition. Various mineralizations were studied in thin sections and by means of the scanning electron microscope. Wood-opals appear to show a structure preservation that points to replacement of cell walls by silica. The wood-opals are mineralogically characterized as 'disordered tridymite'. Other mineralizations, such as chalcedony, quartz, carbonates, phosphates and iron-oxydes and -hydroxydes show characteristics of filling-in. Various mineralization processes are discussed.