Thin stone slabs are being used widely for external walls. These slabs are either precast onto concrete panels or installed on walls using mechanical fasteners. The joints between concrete panes or stone slabs are usually filled with a sealing compound for watertightness. In some buildings, the stone surface near the sealing joints becomes dark and shows a regular staining pattern on the external wall after only several months of exposure. Experiments were carried out to investigate the staining. Granite, marble, and sandstone, and polysulfide, modified silicone, and silicone sealing compounds were tested. The specimens were made of three pieces of stone and had one horizontal and one vertical joint. The joints were filled with sealing compound and the specimens were exposed to the outdoor environment. The relationship between stone and sealing compound was made clear by measuring gloss retention, change of color, and surface water absorption at appropriate intervals. The cause of the stains was found to be migration of silicone which bled from the silicone sealing compound onto the stone surface near the joints. Dirt and soot were absorbed on the silicone, making the stone surface unsightly. -- AATA