Natural stone slabs will fade if left out in direct sunlight. Ultraviolet radiation can cause you to get a sunburn, therefore, natural stone exposed to UV lights will cause discoloration. Some natural stones are treated with dyes to enhance their colors. The dyes in the stone will react to the UV rays and the color will fade rapidly. Also, natural stone has naturally occurring pits and fissures. At the quarry, “filler” material is used to fill in these pits so that the slab is nice and smooth. If kept outside, the filler will yellow and sometimes bead-up on the surface, giving a rough texture.