Formation of sticky layers on bed particles has been considered as a prerequisite for bed agglomeration in fluidized bed combustion of wood-derived fuels. The present investigation was undertaken to determine the quartz bed particle layer formation process in fluidized bed combustion of wood-derived fuels. Bed material samples from three different appliances, bench-scale bubbling fluidized bed, full-scale bubbling fluidized bed, and full-scale circulating fluidized bed, at different sampling times from startup with a fresh bed were collected. Scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS) and X-ray diffraction (XRD) were used to explore layer morphology and chemical composition and to gain information on crystalline phases of the layers and coatings. Significant differences in layer morphology and composition were found for quartz bed particles with different ages. For bed samples with operational duration of less than 1 day, only one thin Ca-, Si-, O-, and K-rich homogeneous quartz bed particle layer that has a relatively high K/Ca molar ratio was found. For quartz bed particles with an age from around 1 day to 2 weeks, an outer more particle-rich coating layer was also found. During the initial days of this period, the layer growth rate was high but decreased over time, and decreasing K/Ca and increasing Ca/Si molar ratios in the inner bed particle layer were observed. For bed particles with age between 2 and 3 weeks, a much lower layer growth rate was observed. At the same time, the Ca/Si molar ratio reached high values and the K concentration remained on a very low level. In addition to these layer formation processes mentioned, also an inner-inner/crack layer was also formed in the circulating fluidized bed quartz bed particles simultaneously with the inner bed particle layer.