From buff to rose to brown to bluish gray, the many colors of sandstone allow designers to create buildings that blend in with majestic mountain landscapes or add excitement to otherwise monotone neighborhoods. But not all sandstone is suitable for exterior building walls. Before working with this material, you need a basic understanding of its geology and composition. Sandstone is a sedimentary rock formed in layers from waterborne minerals produced from the weathering of igneous rock. Sandstone (Type I), by definition, consists of mineral and rock fragments containing at least 60% free silica, within the sand size range of 0.06 to 2 mm. Quartzitic sandstone (Type II), which is harder, contains at least 90% free silica; and quartzite Type III), a metamorphic rock, is the hardest quartz based stone, with at least 95% free silica. Today, most of the sandstone quarries in operation are clustered in Ohio and the Rocky Mountain region. Ohio sandstone, available in a multitude of colors, is quarried in blocks as large as 16 tons from formations as deep as 65 feet. In the West, however, a different quarrying process yields rough-cut sand stone products. In these relatively shallow quarries, the stone is layered in sheetlike strata, ranging form paper-thin to several feet thick.