The conservation and reconstruction of stone mediaeval architecture in Greater Poland calls for the distinction of various species of raw rock material and assorted types of mortar. The example of ruin I and II on Lednica Island illustrates the variety of this type of raw material in the foundations and walls, and determines the diversity of the mortar. The application of pétrographie, macroscopic, microscopic and X-ray diffractometric methods made the task in question possible. Ruin I and II on Lednica Island contain quartizite sandstone (53%) which dominates over gneiss (23%) and granites (8%). Gypsum mortar has been found in ruin I, and gypsum, gypsum-lime and lime mortars — in ruin II.