Sandstone is commonly used in masonry because of its ease of carving, quarrying, and durability. The purpose of this study is to characterize the mineralogy of masonry sandstone from the quarries used to build the Angkor temples of Cambodia, the iconic brownstone buildings of the northeastern United States, and the pre-1906 campus buildings of Stanford University, and relate this mineralogy to the weathering of the sandstone. Samples were examined by thin section petrography, X-ray diffraction, and electron microprobe analysis to identify the composition of the sandstone matrix and cement. X-ray diffraction of the clay fraction proved most helpful at characterizing weathering. Mixed chlorite-smectite was found in the Kulen Mountain, Cambodia sandstone and Portland, Connecticut brownstone. The Stanford sandstone contained mixed illite-smectite. Smectite is highly susceptible to swelling. The presence of swelling clays in the sandstone matrix contributes to damage in the built environment through constant wetting and drying.