The mineral composition, texture, and structure of efflorescences on surfaces from a variety of sources are given. The seeping of water along internal concrete or mortar surfaces produces deposits of calcite; percolation of rain water through the bulk volume produces deposits which may contain trona, aphthitalite, gypsum, and epsomite. Migration of salts from adjacent materials yields efflorescences of alkali sulfates, gypsum, and sodium chloride. SEM studies of the structure of efflorescent encrustations shows that crystal growth tends to occur preferentially at the substrate-to-efflorescence interface, with new crystals extruding previously formed crystals outward. Some implications of the composition of efflorescences for the conservation and preservation of building stone are discussed.